Adding a checked bag mid-itin?
 

I am flying PDX-ORD-ROC-EWR-TLV

As a 1P i know i can check 3 bags. IF I check 2 in PDX, would i be able to add the 3rd bag in EWR. I have some stuff i am going to try and get during my layover.

For United I think the answer can be found in this earlier post.

However your EWR-TLV flight will be on CO, so your mileage may vary.

This won't be a problem at all. You can just tell the gate agent you would like to check it to your destination instead of carrying it on. If you were already at your free bag limit, and I know you sated you won't be, and the flight looks full, they will most likely even waive the extra baggage fee.

Since the OP will be flying on CO, not UA, it might not be a bad idea for him to check with someone like SFOGATE from the CO forum. She's a CO gate agent at SFO and would know all the rules.