If you have IGRN say level 2, then it combines the levels of your 3 highest research labs??? so say you had a level 12, level 8 and a level 7, your research would be conducted in the amout of time it would take a level 27 research lab to do it. :) If you have IGRN, or are planning on researching it, then by all means, research labs on more than 1 planet is a good idea.