Maybe not feet... but I've found that a well-timed back- (or shoulder, or neck, or hand) rub can save a long tech week from becoming too painful!
But really, what an assistant does depends hugely on the context. If you have a large cast or lots of split calls, turning over the sign-in sheet can free you up to be near the director, choreographer, musical director, etc. Likewise, having ASMs handle prop details lets you focus more on the onstage action.
I worked recently with a PSM who asked that all the stage managers keep one another up to date on EVERYTHING - sounds completely obvious, but if your ASMs are new (to stage management in general, to assisting, to this context, etc.) then stating it mildly can make a world of difference. Having assistants in rehearsal can double (or in your case, triple!) the eyes-and-ears you have to collect info. Use them to check in with the cast, designers and crew - are the actors happy? Do they need anything to streamline the rehearsal process? Do the designers need any additional info - details on the director's preference for a special cue, or an extra costume fitting? Does the crew need to schedule an extra work call? Your assistant can take care of smaller phone calls, while you keep your concentration on the action of rehearsal. After rehearsal, or on a break, you can swap/copy notes and keep track of how things are coming along, without anyone's work on the show being compromised.
Best of luck, and keep the SMN community posted on how the show goes!
Meg