As an (amateur) piano technician and pianist, I can say that they would be appalled at how stiff and heavy, and oddly designed, the modern piano keyboard is.
Bach is known to have played on Silbermann's fortepianos and criticized its heavy touch. Beethoven is known to have sent his Erard piano to a local maker to be extensively altered to lighten the action and reduce the key dip. If Beethoven was displeased with a contemporary French instrument (which were heavier than Viennese instruments but still lighter than ours), how much more objectionable would he have found our pianos?
During Mozart's time, the key dip of fortepianos was roughly 5.5mm. In the early 20th century it was 9.5mm (3/8 in.). Now the average is 10.3 and some instruments even have a key dip of 11mm!
Bach, Mozart and Beethoven would complain that playing on a modern piano is like having your fingers swallowed.
During Mozart's time, the touchweight for minimum sound at middle C was about 31g. Nowadays it can go up to as high as 110g. Most manafacturers even put lots of lead weights in the front of keys to compensate, but this actually makes the action heavier (not lighter) when a certain dynamic level is exceeded, due to mass (hence inertia) being more significant than weight (i.e. gravity) after a certain amount of acceleration.
Bach, Mozart and Beethoven would complain that playing on a modern piano is like wrestling a monster with your fingers.
From Mozart's time up to at least 1850 (or even later), the octave span of piano keyboards was 159mm. (The octave span is measured from the left edge of one key to the right edge of its seventh, including the gap.) During Bach's time many harpsichords had a narrower span than this. Now the modern piano always has a span of 165mm.
Bach, Mozart and Beethoven would complain that playing on a modern piano is like playing with your fingers forcibly splayed out.
During Mozart's time, the accidental keys only protruded about 9mm or less above the natural keys. Today it is as high as 12.7mm, requiring you to lift your fingers higher to play the sharps. Moreover, natural key fronts during Mozart's time were only 38mm long, meaning that the accidentals were closer to your fingers; by contrast, natural key fronts today are 51mm (2 in.) long, increasing the front-to-back distance needed to get to the sharps. The high and faraway sharps make for inefficient playing.
So they would complain that playing on a modern piano is like playing on two separate keyboards on different levels, one white one black.
During Mozart's time, the natural key tops were ebony. Today, they are plastic. Wood absorbs moisture and plastic doesn't. When you play on wooden keys, your fingers do not stick to the keys due to sweat and grime from your fingers (or leftover from other people's fingers). Ivory also has a similar quality.
Our three friends would agree that the modern plastic keys are slimy and too sticky to play.
So far we've talked about the keyboard and action alone. If you're also talking about the sound of the instrument, they would have plenty of negative things to say.
They would find the murky, stodgy bass very disagreeable. They would not be able to play closed position chords in the bass anymore, which their compositions use so often (e.g. the opening C minor chord of Beethoven's Pathetique - how effective is that, really, on a modern piano?).
Mozart in particular would find that the piano has too much and too little dynamic contrast at the same time. During his time, if you hit a key harder, the sound got a little louder but also brighter; if you played a key softer, the sound would be a little softer but also mellower. In today's pianos, you can play very loud but the sound won't be very bright, and you can play very soft but the sound won't be very mellow. As a result, he would not be able to realize the louder dynamics and accents in his music because on a modern piano he would merely get a louder, booming sound without getting more brightness.
They would not be able to realize their articulation because the modern piano only allows legato playing (staccato or detached notes don't sound good on a modern piano, due to long sustain and dull attack). Mozart out of the three would have the most problems because his music is the most clearly articulated of the three. On a modern piano, you have to 'change' Mozart's music and use legato where it is not originally required, or hold notes longer than they are written.
The murky, inharmonic quality of the modern piano, combined with its long sustain, also limits the use of the damper pedal, which Mozart used sometimes and is vital in Beethoven. Because the modern piano is so sonorous, there are times when we can only use half pedal (or none at all), or change pedal more frequently, than what Mozart or Beethoven intended. The first movement of Beethoven's Moonlight sonata is actually meant to be played entire without dampers, that is, with the damper pedal depressed throughout the whole movement. (Also in the first section of the last movement of the Walstein.) But on a modern piano one can't do this - you have to change pedal with each harmony to avoid creating a mess.
They would remark on how homogenous the tone colour is across the modern piano's compass, but at the same time note the incongruous break where the bridge is separated into treble and bass (and steel and copper-wound). Bach would not like how the voices in his fugues would get blended together. Mozart and Beethoven would not like how the different 'personalities' of the piano's different registers have became more alike each other - it would seem bland. (The tone colour of modern piano is homogenous overall, but a noticeable break occurs; in a fortepiano, the tone colour is very different across registers, but there is no noticeable 'break' because the individual transitions are smooth and gradual, only becoming noticeable when totaled.)
They would also dislike the cold, metallic tone of our pianos, and preferred the warmer, woodier tone of their own time. They would not like the 'delayed' quality of our tone, in which a tone 'blooms' gradually into a full sound (their own pianos gave instant and direct tone). It would force them to play too slowly.
These are what our three friends would have thought, but it's also what many modern musicians and audiences think. I guess that's why the fortepiano, harpsichord and clavichord have become more and more popular over the past few decades.