A 5/1 ARM is a mortgage that:
a) has a fixed rate for the first 5 years (in your case 3%) --but--
b) after 5 years it can adjust annually. The rate adjustment is based off the LIBOR, Treasuries, the WSJ rate, etc. The terms of the adjustment are negotiable and are written into your note (i.e. at the 6th year, it will adjust up or down by a maximum of 3%, just for example purposes).
Here is the problem.
a) There are 15-year fixed products on the market right now for not much more than 3% (3.5 seems about average).
b) With an ARM, in year 6, if interest rates are at 11% (right now for example the WSJ prime rate is 3.25%), then your rate will go up to 6%. In year 7, if the rates are still at 11%, your rate will go up to 9%, and so on, until they meet whatever index drives your rate.
Your rate could also go down (which is why these are seemingly attractive). ARMs were super popular from the mid-90s until 2004-ish because interest rates were really high. It wasn't uncommon for the fixed rate to be 7-8-9%, and that made a 6% ARM look really attractive. It was even more so when the prime rate dropped out, many ARM holders had their P&I go down during that time, but are just as exposed to it going up when interest rates eventually go up (which they eventually must).
For me, I would not get an ARM right now unless you plan to stay there 5 years or less. The lending rates are being artificially suppressed by the Fed and that situation is not going to go on forever. At some point interest rates must rise.