Just a week ago I put a chainguard on my girlfriend's bike. It's a triple with similar chainring sizes to those you mentioned.

The chainguard came off an old singlespeed and is the type that attaches to the seat tube (near where a front derailleur normally attaches) and also to the seatstay. I cut off the back half of the chain guard and kept the part that covers a the top of the chainring(s). It seems to work pretty well just like that- the part where pants would hit the chain is only the forward top part of the chain's loop.