Apparently a Triumph T plane is the same as a cross (X) plane 4 cylinder crank except missing one of the crankpins.
So looking at it from the end it looks like a T, say cylinder #1 crankpin is at 12 o'clock, cylinder #3 will be at 6 o'clock and cylinder #2 will be at 3 or 9 o'clock.
See the illustration on the top right here;
The only images I have of the 2458cc Rocket 3 (two below) look really like 120° angles between crankpins.
I am pretty sure they are 120 degree, they are too smooth for T plane. They also don't sound like a T plane, the offset firing order has a V8 sound to it, sort of.
The T plane also came out after the rocket was designed, the T plane is a pretty recent edition to the lineup.