Female Blackbird lost mate and has 3 fledgling chicks around 3 weeks old

They can. Obviously, assuming they make it out of the nest, she will struggle to get them to independence, but should be capable of getting them so far, esp with you helping.

They have left the nest 5 days ago but can’t fly very far.

In reply to Michael Ridyard:

Doing well so far in that case.

Good news the blackbird male is back. He must have been taking a break. Only 2 fledglings now but the hen is busy repairing her nest for the next brood. I’ve been after a garden friend for awhile now to help with my slug and snail problem and they are the perfect solution.

Might well be a different male.

He’s feeding and protecting her 2 fledglings. Would a new mate do that?

Getting slightly confused. When did the young leave the nest? From what you have written, the male was absent for 11 days. The young have been out of the nest at least 7 days. They are all (bar one lost juvenile) still in your garden?

The fledglings are 4 weeks old now and still being feed by their parents although they are starting to peck at food now. Yes they are still in the garden.