I have read that as soon as you see the little tendrils sticking out between
the leaves, you should have something in place for them to grab onto, or
they might not ever grab. I have the 2 foot chicken wire going right now,
and the peas are at the top of the fence, blooming and producing as I write.
If yours are only 3 inches high now, you are either in a very cold
climate, or you got them into the ground a bit late (they should be started
in March in most of the cold climate parts of the US - earlier in warmer
climates) . The problem is that they will begin to give out as soon as the
weather stays consistently hot.
When mine were about 3 inches, I took the spring pruning clippings from
my butterfly bushes (lots of 3-4 foot lengths), put them in the soil near
the peas like a picket fence, then braced the chicken wire in and around
them. So far it has worked pretty well. No slug holes in the peas yet.....