really new at this

my plants are about 3 weeks old I've got the third set of leaves emerging. The plan has beento keep them short and bushy if I can. When should I first pinch them back to develop more tops?

Look into Low Stress Training before you begin topping. You may find that way of growing gives the ability to better control how and where the plant grows. Topping at this time is fine and will keep the height down - Thing is you don't want to get it too bushy by topping too much.

the 4th node,do try as final phase said LST that girl,when mine was around 8" tall or smaller I pulled the main stem down lower than the others branches and tied it down to hold it in that position.. I used small wire, then it opens up the middle and all sorts of stems fight for apical dominance and you get excellent growth this way ,and pull down and tie down all the stems to hold them in place ,and let them grow side ways and all sorts of stem pop up ,and later on become bud sites,and yes[ fem ]them also, hope this helps

Wait until your plant has five nodes, then top above the third node. Remember that topping doesn't necessarily keep your plant short and bushy! It's best to use it in combination with other training methods, like LST. I myself usually use super cropping instead.

I grow mainly autos so not alot of topping for me, however i use LST for all my plants and i start as young as i can. As long as there is something to LST i do it. Probably around early week 3 like you, but a picture of your plants would help us determine that better.

When i do top, i also try to top early as possible aswell. Lets see some pictures so we can help you out.

These are my girls. (well that how I refer to them. Positive imagery, right?) at a little more than 3 weeks.