as long as you prep the male adapters properly, and the ends of the copper, it won't leak. Read the manufacturers guide for the pro press fittings. You can also buy 3" PP ball valves. They cost more, but will save a step.
is the best invention for large fittings. use a 3 pp ball valve, don't use adpt. unless u r using the adpt. to take up the difference so u don't have to buy a piece of 3" copper and coupling. the best way to clean off the solder is to heat it up then wipe it with FIBERGLASS INSULATION not a rag, trust me. u will be able to wipe it so smooth u won't even need to sand it after, but i do recommend sanding it. good luck