Consider that three of you using the connection at the same time may well increase latency to a noticeable level as can anything else going on in the background (sky box downloads a program it thinks you will watch to cache, windows update, virus scan etc.). Powerline quote full duplex speed, so 1200mbit is a theoretical 600mbit either way, realistically you were probably getting well below 1200 to start with and you need to divide what you do get by 3 if all users were maxing the connection at the same time. Gaming tends to be low bandwidth but can be highly sensitive to latency depending on the type of game and how good the player is.
Best option (and the only one I can really recommend) is ditch the powerline and run cables, this will give you gigabit each way on each device and removes a variable from the equation. If you have to use powerline, then investigate the sync’s, make sure you followed the obvious best practice (no surge suppression, master socket rather than a spur, same circuit, no extensions, latest firmware on each adapter etc. and check the syncs in the controller app).