What is the purpose of these on 3pt hitch
 

https://polr.partstree.com/v1/iplima...I7aToxMDAwO319

Part #3, two pieces, one on each side of rear drawbar support plate. Part #3 is called a Cam, Sway Limiter.

So that the lower arms do not swing in and snag when raising.
http://www.tractordata.com/ltphotos/...1235-td3b2.jpg

So nothing hooks to them, they are just there as a bumper for the arms?

That's correct