What kind of filtration are you looking to enhance?
I've got a 150G (72" x 18" x 28") with an Eheim 2217 and an Eheim 2028. I put prefilters on the inputs of both and water is clear. I added a spongefilter on a powerhead to get some additional current flow and give me something to toss in a QT when needed. Between the two canisters and the sponge, I feel like I've got plenty of mechanical and biological filtering. YMMV.
Just my opinion, but I think you'll be well served with three 2217's. I will say, however, that maintenance is much easier on the the newer Eheims like the 2028 just because of the baskets. The 2217's are a great value, however.
Good luck.