For a detailed pictorial guide on replacing RTC batteries, refer to the guide here.
Leaking Batteries ~~~~~~~~~~~~~~~~~ There is a battery on the Amiga 3000's motherboard which keeps the clock up and running, plus keeps the parameter RAM set. This RAM maintains things like the SCSI controller ID, Synchronous bits, Timeout modes etc. Many of these batteries have started to disintegrate, not only on Amiga 3000's, but on A2000's, A501's and A4000's. No system is free from these issues: Quickly check if your battery, often blue or red in a little barrel shape on the left side of the A3000 motherboard, is leaking! If it is, you'll see little flecks of white acid crystallizing. Many users even reported that the battery had already started leaking onto the motherboard, corroding the copper traces. My own personal A4000 already had the battery leaking. What to do? Immediately cut off, if not replace, the battery. NOW. Do it. Many saw it, put it off, and later had their motherboard torn apart and destroyed by acid because they didn't fix it fast enough. To remove the battery, snip off the legs (in three points). This is recommended if you can't replace it immediately. Be careful about damaging chips around it. You may wish to snip the rear pin and rock the other two back and forth until it snaps easily. This may even be a better idea if you plan to replace it immediately-at least you won't get bits of acid all over your board. To replace, it's highly recommended you desolder the battery. First, cut the battery off with a pair of snips. Then, unscrew the motherboard. Yes, this is a pain in the ass. There are tens of screws and hex pins. Once you've removed it, desolder the remaining legs of the old battery. Use a low power 15W iron with a very small pencil tip. You may not need desoldering equipment, just heating the old solder up may let the old bit come out easily. If this doesn't work, get desolder braid or a sucker. Once all three old pins are out, put a new battery in place. Replacement components are easily found at PC stores, electronics shops, and some surplus stores. You'll want a NiCad 3.6V 60 mAH component, although nearly any 3.6V NiCad battery will work. Ask especially for the three legged model. The two legged kind are electrically the same, but a pain to put in. Do not use Lithium batteries, these do not recharge and could cause an explosion! A good idea is to bring the old one in and ask for a direct replacement. Of course, make sure you package it before you leave-use an old baby food jar or something. Battery acid is a bad thing. Other suggestions, include using cordless phone batteries and the like. Use at your own risk. If you used desoldering equipment, the clean holes should present no problem. If you're cheap and you didn't, then try this idea-put the battery in place, then heat up the old solder on the bottom side of the motherboard. Apply a little pressure to the battery. Physics should suck the leg into place with solder around it. Repeat with each leg. Start up your machine, and let it run for an hour or so to charge up the new component, then shut down. Power up to check if the date and time are retained. As for disposing the old battery, package correctly, and return to your appropriate government recycling center, or even your local electronics store like Radio Shack for proper disposal. It may be illegal in some places to put them in household garbage. Feel free to curse at the sucker before you toss it.