I avoid betting on under / over but I like this one, line should be closer to 3.5 goals.
San Jose and Portland have some of the most fragile defenses in MLS (San Jose exactly the worst) conceding more than 2 goals average per game. Counting the "MLS is Back" tournament also.
Sometimes San Jose make lowscoring games (like 0-0 with Galaxy in the weekend where both teams looked decided to take a day off and do nothing) but usually there are too many goals when they are playing, from 5 up to 7-8 in one match.
Portland are similar profile, just one of their last 12 games had less than 3 goals (1-1 vs Cincinnati in the "MLS is Back" playoffs), didn't manage to keep a clean sheet in any of these games and only once failed to score (0-3 against Seattle).
Meanwhile both are scoring normally. e.g. San Jose found the net once or more in every of the 8 games before the mentioned draw with Galaxy, should be 3 goals at least.