Shower/tub dumping water help before house becomes Atlantis

I live in an older house with bad ventillation, crappy pipes and so on. My tub/shower combo has 3 knobs: hot water, shower, cold water. It is dumping water at incredible rates...not just little drops. How can I fix this? I need the idiots guide please, as I will be doing this myself. Thank you.
PS...if I can't get this fixed soon, I fear my house will suffer the fate of Atlantis

Is there a brand name on any of this? That would help .

Is the water coming out of the tub filler, or out of the handle?

Sometimes all you need is to replace a washer, but sometimes the seat (the metal part the washer pushes up against to shut off the flow) needs to be replaced as well. Sometimes, it is just easier to replace the guts of the faucet. And, what may be the best solution, is to replace it with a new one! This can be messy, since it usually entails some new tile work.

The guts of many faucets can be replaced. You'd need to turn the main water off, then take the guts out of the faucet to a good plumbing supply house (you MIGHT be able to find the guts at a place like HD or Lowes). By having the old parts, you can match it up with new ones. This takes some skill, but once you have the new guts, it is pretty straightforward replacing it.

To take (most) faucets apart, you turn the nut that surrounds the handle (called a packing nut) counter clockwise until it is loose. Then, you turn the faucet handle as if you were going to turn the water on and it should back out of the guts of the faucet. Take that part to the store, and get a replacement. Put it back in in the reverse order (have the guts so that it is not fully seated while you screw down the packing nut). Note, the new part will not usually include the handle, so you need to either remove your old onces, or buy new ones at the same time as you will not be able to get the packing nut off to reuse unless you take the handle off.

One of the pros may have some other ideas...

faucet

You have to remove the handles in order to remove the chrome "trim piece" which is between the handle and the wall. Very few stems remove by unscrewing the packing nut. Most require that you unscrew the outer part of the unit holding the stem.

To replace parts in the
Price Phister faucet,

Make sure the water is off at the main shutoff.

1. Remove handles with a faucet puller. Remove trim.

2. Remove faucet stems with a socket wrench.

3. Remove seats with a seat wrench.

4. Replace seats with pipe sealant on the threads.

5. Install new valve stems.

6. Replace trim and handles.

p/p

I just had one yesterday where the repair person did all of those steps. The customer could not turn the water to the washing machine off, and neither could I. Rather than loosen the packing, which would have eventually led to a leak when the valve was turned on, I added and changed some of the steps listed. First step is to buy a new Price Pfister faucet. Then follow the steps donw to t #4. The new #4 would be as follows.
4. Remove all parts from the new faucet.
5. Install all parts from the new faucet in the old piece in the wall.
6. Enjoy you new faucet that has a "half turn" from on to off, will almost never leak at the handle or out of the spout or shower head, will almost never get too tight or too loose, and has a lifetime guarantee.