Below is a quote from Big Dean. He used to post here before he died. You didn't need to question his info.
"The firing order for the AG3-152 Perkins 3 cylinder engine is 1-2-3 distributor rotation is counterclockwise points gap is 0.021 plug gap is 0.025"
Number one on the distributer cap can be found by rotating #1 piston to TDC on the compression stroke. Then just remove the cap and see which tower the rotor is pointing to.