in my experience (i worked at kinko's, a photolab, and an ad agency for many years), most large format printers will be fine at 150dpi. any lower than that and you start to lose detail. prints done on smaller printers (letter or tabloid) will usually look better at 300 dpi.
basically, your resolution should be double the line screen of the printer. the line screen is how many lines per inch the printer can handle. don't count on your average kinko's employee knowing that information though.
another consideration is that a 300dpi image at 3 feet by even a foot is going to be a huge file. if you like the amount of control you have over pixels at 300 dpi, i would suggest building your file at half size, ie 1.5 feet by .5 feet at 300 dpi and then when it is finished, you can bump it up to 3 feet by 1 foot at 150 dpi and have no loss of quality.
my only other suggestion is that if the file size becomes to cumbersome, you can always slice up the image into separate pieces, for example once you have the ocean set, break it up by continent into smaller separate files. continuity between the pieces becomes an issue, but as long as you have your style well in hand, it shouldn't be an issue.
my only other advice is when it comes time for printing, ask to see a test strip first. it will save you a lot of hassle if for some reason the colors are not what you wanted. most places charge by the sq ft of paper used, so you want them to get it right the first time. its amazing what adding or subtracting a little cyan or magenta will do to a print. my guess is you'll be building the map in RGB, but printers use CMYK which has a smaller color gamut. stay away from super saturated colors, especially blues and greens.
hope that helps and good luck!