Our married JT rtn 2024 MAGI is <$160K. She has $3K 2024 "wages"(fully retired for 2025)& takes RMD frm 403b. May I contribute to either my SEP or an IRA for 2024?

You can only contribute to any kind of retirement plan up to your "compensation" from working.  You can contribute even if you are older and even if you are "retired" as long as you have compensation.

Your SEP limit is 25% of your compensation, so it would seem like your compensation should be around $16,000 for you plus $3000 for your wife.  That would support an $8000 deductible IRA contribution for yourself and also up to $8000 to an IRA in your wife's name, using the spousal IRA contribution rules. 

You can always contribute to a traditional IRA, as long as you have compensation.  Whether you can take a tax deduction for those contributions depends on your income and whether you or your wife was "covered" by a workplace retirement plan for 2024.  And if you wife worked just a few days in 2024 but was "covered" by the 403b while she worked, then she is covered for the whole year.  The income limits for the tax deductible IRA contribution are here.

https://www.irs.gov/retirement-plans/ira-deduction-limits