In order to find out the life cycle and host requirement of fleas, I'd contact a local pest control company and ask the question. I do find it quite suspicious your present tenants lived in the house for three weeks without complaint. A lot of "living" can occur during that time including a visiting pet with fleas!
I think I'd let it go this time around. Should it happen again, I would inquire with them about their living habits (e.g. do they go camping, have visiting pets, etc).
Best of luck to you!
"Do all the good you can. By all the means you can. In all the ways you can. In all the places you can. At all the times you can. To all the people you can. As long as ever you can." John Wesley
When called by agents to show my listing permission was preconditioned on not visiting that house before coming to my listing.
Now, are your tenants co-operative or combative. If you choose the path of 'dong what's right' are they going to punish you further? Meaning, are they going to attempt to stick you with the bill for alternative housing during remediation? Cost of dry cleaning and washing of dishes after event? Are they going to hold you the cause when headaches and allergies flare up in the next month or two?
If you have trustworthy folks on the other side, have the house fumigated. If not, have the tenant take care if it themselves. You place was flea free when they moved in.