Hello, I'm Nathan and I will be assisting you today. Please remember to rate/accept my answer when we have finished. Thank you.
HI there, sorry to hear you're having trouble with the tv.
Water and electronics never mix well, so I don't want to get your hopes up.
The 3 flash error means that the lamp (bulb) is not seated properly, or that the door to the lamp housing is open.
This could truly be the case, or the microswitch/sensor, or one of the boards that detects these things could be faulty.
If you're handy, you can do some basic checks yourself to try and fix this. Beyond that though, for a set this age, if you had to get it repaired professionally, likely to replace a main board that had been shorted out by water damage, it probably would not be worth the cost of repairs.
Before doing anything, you might want to consider getting in touch with the seller, and letting them know you have discovered the water issue, and that it won't turn on, to see if you can get your money back so you can invest it in a different tv instead.
Otherwise, go ahead and open up the tv by removing the rear cover. There are several screws you'll need to remove, but they should all be obvious.
What you're looking for is the lamp housing, which looks like this:
check that the lamp is inserted securely, and that the door on the housing is shut tightly.
If that checks out, then the set likely has a bad switch, or a bad board, and is going to need repairs.