We have been in exactly this boat last summer and again this summer.
Last year, #1 could ride well alone and #2 was on the Trail-a-Bike with my husband. That left #3 (a toddler) and twins who were 6-9 months. We puzzled about bicycling for a long time. The best we could figure out was that we should build a little platform like a classical chariot or a kiddie board for a stroller and attach it behind our bike trailer. Needless to say, we somehow didn't manage to find time for such a construction problem and it was a summer without biking.
Now we have #1 and #2 biking on own and #3 on the Trail-a-bike with my husband. I take the twins in the bicycle trailer. Same problem you have: what to do when Dad's not home?
: I have a couple of times squashed all three boys into the bike trailer when desperate. I strapped the twins in as usual. I sat my 3-year-old sideways in the footwell. He's mature enough to stay seated when told. His head was a little squished by the front snap-over net, but he was happy. The twins were kicking and pinching him a bit, but we all had fun anyway and the grocery trip got done.
You could also consider an on-bike carrier for one twin and then put the older child and the second twin in the trailer. This was not an option for us as the rear-mounted bike seats make me SOOO nervous due to the instability they create for the adult rider. The front-mount seats are really expensive and we weren't prepared to spend that much just to try it out since they appear to be quite uncomfortable for the adult since knees must be spread around them while pedaling.