I have the same situation with my kiddos. They are all toddlers and ALL need to be carried up to the bedrooms for their naps. I do it one by one and leave the others in my completely childproofed, gated playroom. Each of my kiddos is trained to go right to sleep so I am only a minute literally each time I put one in their playpen.
I have also on occasion, done naps immediately after our walk to the park. I keep the others buckled in their stroller/wagon and run into the house with one at a time to put them in their beds.
I know its not the greatest situation but unfortunately, there is only one of US. Parents would be doing the same thing if they had multiple children of their own.