Lola, sorry, I've been on vacation, just got back on-line. The 25 or 29/30 gal size aquarium will do fine for your 3 plecos. No need to get something huge for what you want for the winter only, then planning to put them back into the pond.
FYI, I had a 6" pleco I put in my pond last summer, it grew to 12" or more (went side to side and then an inch on a 3.5 gal bucket). Sold it, as I didn't want to put it into my 29 high gal aquarium which I had set up for tropical. I now have a very small albino pleco in the goldfish pond, and he has not grown at all like the larger common type, so it's maybe 3". It will have to come in, and will go into my 55 gal tropical aquarium. I bought a 55 gal for $45 on Craigs List, so watch for them, people have them all the time really cheap.
As far as filtration, use normal type of aquarium filtration. Although plecos DO produce more waste than most tropical fish, they don't produce any more than goldfish, so not sure why Capewind is suggesting such a large container for 3 - 5" fish.
Once you get the water bacteria started (BTW, you can use your pond water to get it seeded, if that is all you will have in the tank ...), there should not be much in the way of water changes. I have a 55 gal with about 15 fish ranging from large angels (2), killifish, assorted plecos (3) and cory cats (5), platys, rummy nose tetras and another tetra type, and 4 or 5 loaches. I do vacuum the bottom only about once a month, when I add water when the level gets low. I'm a firm believer that if you have the correct bacteria going, water changes are not as necessary. However, that being said, I know most do much more often water changes, and it's needed. My water is crystal clear, anad have not lost any fish in a very long time.
So ... if all you are looking for is a place to house the 5" plecos, a 30 gal would be plenty in my opinion. If you are looking for cheap, find any type of tote for cheap, set up some type of hang-on-the-back filter and you're set. Or, use bubbler, and vacuum the waste from the bottom once every week or two. Voila.
I had a kiddie pool in my basement last winter, as I purchased 14 small koi (4-6") and didn't want to put them outside in December when they were delivered. It worked well, looked very rinky dink, but all that mattered is that it worked.
And, I'm pretty sure plecos are not prone to jump, but maybe others have had them jump. They tend to be bottom dwellers. Now, if you put koi in a shallow kiddie pool like I did, they definitely need a net for the first few days when they are likely to jump from fright. Once they settled down, the net came off and I never lost one.
Good luck!!!