No, you won't have any issues with vignetting - the 35L has a big safety margin there, my tests
show that you can stack three standard (F-Pro) filters with no increased vignetting.
The problem you will have with a 72-to-77mm step up ring on the 35L is that you cannot mount the lens hood over the step-up ring (I just tried it), nor can you really screw in the ring/filter after mounting the hood (maybe if you iron your fingers flat first). It's rare that you can use both hood and a step up ring (possible with 77mm filters on the 85L II, but not on any other lens I've tried). So, you'd need to choose between filter and hood with 77mm filters on the 35L.
Definitely get a set of filter wrenches - pairs of little plastic grippers, just search "filter wrench" on B&H or Adorama, they're ~$5 a set, you need the right range for your filter sizes.