There is The AirTrain that shuttles between all 3 terms as well as other places within EWR. You exit the secure area and then need to look for the signs, and go up 1 level with an escalator, then be careful to get on the train heading to T-C and not the opposite direction

Yes you will need to exit the secure area and then once at T-C go thru TSA

Time to allow will depend on the lines at T-C = depends the time of day and if you have Elite status (although lately I have found that the Elite line is longer and moves slower then the reg line).

As of now thats probably the best way to go, maybe UA will eventually have a shuttle bus.