Practicing 3x3 is definetaly the most helpful thing you can possible do. But since the megaminx color scheme is different than the 3x3 knowing what pieces to look for helps a ton especialy after the first 5 f2l's. Knowing multiple algs for the last layer helps a lot too. You can use J and T for 3 out 5 edge permutation cases (i think)
Well it depends on the 3^3 method you use. Practicing 3^3 doesn't help me at all for megaminx
Anyway, you don't need to be superfast at 3^3 to get decent averages at megaminx. Just try to improve your recognition on the dodecahedron specifically.