Very hard to answer.

Suspect your are looking at the seatmap (saying "3 out of 8 open"). This may be incomplete. Need to look at Fare Code availability, Fx, what is x?

Since UA will be trying to sell the F seats, UA will wait till close to flight time to open up the upgrades and that means you may be at risk of a higher status passenger getting the upgrade. Plus you do not know who else is presently on the waitlist or might waitlist before Saturday.

EWR/SFO flies A319 (and sometimes A320) and can be a tough upgrade.

So, when? if at all, at the gate would be my guess