Most boards that use Speeduino code are designed to fire multiple injectors per INJ channel. You should check with your specific board's specifications for the limits your board version can handle, which is based primarily on the copper trace size, but the MOSFET capability may be a factor.
For example, the v0.4-series is stated as capable of 8 amps maximum average on its traces, and in-fact there are many V8 engines running 4 injectors on 2 channels, and 4-cylinder engines running batch fire with the same channel loading, as real-world examples.
Run your calculations for the average amps you will place on your injector channels, and check your board design for ability to handle that load. Example calc with 12-ohm injectors on a V0.4.x board at 15-volts:
: 15V / 12ohms = 1.25A
at max voltage, each
Avg Amps per Channel: (1.25A * 3inj) * .90maxDC = 3.375A
average at 90% max duty cycle
Channel load %: (3.375A * 100) / 8A = 42.2%
channel capacity (rounded)
In this particular example, you would be fine for 3 (or more) 12-ohm injectors per-channel using a v0.4 board.