If your husband was a baptized Lutheran there is no requirement for him to be baptized again. The absolute longest RCIA lasts is a year, not three years. And that’s for a person who has to be baptized, which would have not been your husband. His RCIA could have been a month or two, or a week or two, getting some instructions from the priest, not an RCIA group. Three years RCIA....never.
Actually, for all the stages it can take years:
The Five Main Stages of the R.C.I.A. The Period of Inquiry...This period lasts as long as the person needs it to last, from a few months to several years, if necessary.
The Period of the Catechumenate...This period, too, lasts as long as the person needs it to last, from a few months to several years, if necessary.
Who needs to attend RCIA? People who have not received the sacrament of Baptism must attend RCIA for an extended period — ideally, a year, and preferably longer. - https://udayton.edu/ministry/liturgy/becoming_catholic/five_stages.php
Who needs to attend RCIA? People who have not received the sacrament of Baptism must attend RCIA for an extended period — ideally, a year, and preferably longer. - https://www.ctkcc.net/rcia/rcia-frequently-asked-questions-2/
And yet we have SO many that FR Catholics claim are poorly catechized.