from the IRS website https://www.irs.gov/taxtopics/tc412#:~:text=You%20should%20receive%20a
You should receive a Form 1099-R PDF from the payer of the lump-sum distribution showing your taxable distribution and the amount eligible for capital gain treatment.
so you need to contact the plan to find out why box 3 was left blank
Only the taxable part of the distribution from participation before 1974 would qualify for capital gain treatment(if you qualify)
