I have a Yamaha ydp-121 88-key piano. It has 3 keys that are sticking. Any ideas how to fix it?

Suppliers here

https://www.google.com/search?q=yamaha+model+YDP+161+piano+board

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You will need to gain access to the keybed, remove screw from back of keybed the fix it to the base of the keyboard. Slide the keybed back making a gap of about 2cm at the front of the keybed.

Look at the back of the key towards where it pivots and you will see a small arrow, push something like a credit card between the keys at this point and the key will pop up, you can then slide forward releasing the key.

If you need more info please ask.

A YDP uses the same key type as a basic Yamaha Clavinova. As you have stripped the keyboard to reveal the keys take a look at the end where the key hinges. There will be an arrow pointing to one side. You will need to remove att the screws holding the key base (black plastic ) to the wooden base. Slide the whole keyboard back a couple of inches, push down where the arrow points between the keys, the offending key will pop up. Slide forward and jiggle out. Replace key. reassemble. If the tag on the back of the old key has snapped off remove the piece of debris that might be there before refitting.

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replacement keys can be found online

http://www.beckettsmusic.co.uk/shop/scripts/prodView.asp?idproduct=17431

On SOME Yamaha pianos, there is an electrolytic capacitor 1mfd/50v that becomes leaky and causes this. On those with the particular main board the capcitor is at location C94. On other pianos there are other caps that bypass the wire from the pedals that are ceramic. Likely one of these has shorted. There is also a possibility that the pedal contact has shorted. Test for this by unplugging the pedal cable. If it still sustains, then problem is internal. Unless you are adept at electronics this is NOT a DIY repair.

Yamaha keyboards can be prone to sticking keys due to the rear part of the key cracking around the return spring - if this is the case the only solution is replacing the affected key modules. However this problem generally only occurs with long term use by very physical players.
Otherwise the problem may be caused by a build up of dust / fluff between adjacent keys - try raking out the spaces with a thin piece of fairly rigid plastic (maybe a credit card might work?)

Typically LOTS of screws... concentrate on those around the edge. USE GREAT care and force nothing when removing the cover...and great care of cables between the cover and the base. Mark the cables so you remember the direction they attach.

When you get edge screws out, then gentle lift and see where it seems to be held...

Some of these synths have 50 to take out to get apart. MOST are in plastic bosses and keep track of the size and length as you can put too long a one in the wrong place and crack the boss.

I use a compartmented plastic tray to keep the screws in order.

Check if it might be dragging on the edge of the case. The restoring force for some of these comes from a silicone rubber dome that is oart of the contact. SOMETIMES the dome fails.

you may not need to open it up.
try using a wax based furniture polish on the keys, it will leave a layer to act as a lubricant between the keys.

have you tried using the can of air. you can get it at almost any drug store walmart or kmart. just spray the can of air between the keys and they should become unstuck.