The 3rd wire is only for use with points-triggered ignitions; you no longer need it. You will however have to run a true ignition 12v+ wire for the Painless harness; the original resistance wire in the harness is no good. You can follow it back to the bulkhead connector where it comes from through the firewall and replace it with a 14ga. wire or run a new wire in through a grommet and hook it to the 12ga. pink on the ignition switch.
You can't use the "acc" terminal on the fuse box; it is not on during crank. If you're not using the original oil pressure actuated fuel pump switch, you can use the pink on it instead.
77 Vega Estate wagon- project will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(email@example.com)(firstname.lastname@example.org 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
04 Silverado 2WD ex.cab (work truck/parts hauler)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- older son's ride
99 Saturn SW2- spare
and 2 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....