Except random battlegrounds aren't premades and you can't guarantee any level of communication, individual player skill or ability to function as a team, so the 3v1 strategy is risky because it tends to result in a PUG spreading itself too thin. In a PUG it's somewhat safer to focus your team at two bases, deny the enemy the flag, and then go for targets of opportunity with whatever good players are present. That could be the flag or it could be other bases.

It's not clever, but it is robust - it doesn't put much pressure on your PUG to perform, and it allows a handful of good players to control the flow of the BG by capping the flag/assaulting a weak base while the enemy zergs your PUG defenders/etc.

This is why in PUG EotS I almost always go midfield. If the enemy sends forces to mid, I can prevent them getting the flag for upwards of a minute into the match, tying up their guys in the middle while our numerically superior forces have a better chance at the bases. If the enemy doesn't go mid, I get free control of the flag and am in a position to spot for targets of opportunity. I've seen this strategy work time and time again in PUGs; I've seen 3v1 successfully pulled off a mere handful of times - and each time was with a friendly team that dominated the enemy on every level anyway - and lead to many many failures as it degenerates into a 2v2 with the enemy controlling the flag because our team is too busy trying to cap another tower against evenly matched forces.