l have a TTR125L and every time l jump a 3 ft jump, it bottoms out, what should l do?

You can add more preload to the rear shock, or you could swap out your springs for some stiffer ones, or change out your shock fluid for some thicker fluid.

You may be too big for that bike, or it could be the way your landing. Are your knees bent and are you using them to soak up some of the landing?

Add pre load.

Or buy a decent MX like a CRF.

Unless u know what u r doing l would take the bike to a specialist in suspension as they will get ur weight & then set the bike up to suit it. Everything the fellow above said is correct & u can adjust the fluid in the forks to change the rebound & also get a stiffer spring. You could perhaps look at what size spring u have in the bike & try going up a size & see if that works.
Perhaps if u dont have the money to outlay for someone to do the work for u u could ask a specialist via phone what size spring u should be running for ur weight.
l know we always have to get springs basically shaved down for my son as he is just so light that it is difficult to get a spring for his weight.

there is not much you can do to stiffen the suspension, and a shop cannot do much either..
l too have a TTR 125, and it is next to impossable to find aftermarket springs that will hold anything heavier than a 10 year old.
This is what l did..
take the forks off, drain out the old fork oil, add a stiffer weight, like 15 wt. oil..

as far as the shock goes, try tightening the preload adjuster, and thats about all you can do to it...