What do I check for Jan-March on the HDHP page if my husband had an HDHP along with Med A only? He did not start with Med B until he retired on April 1.

You must select Medicare or None for any month your husband was covered by any part of Medicare.  In this case, you husband is ineligible to make any HSA contribution for 2018 because he was covered by Medicare the entire year.  If you were covered by this HDHP plan or another HDHP plan, your eligibility to contribute to your own HSA is independent of your husband's ineligibility.