Perhaps I shouldn't give up on Box3 yet. Here is a female bluetit doing a full blooded nesting wriggle!
A few minutes later she is in with some nesting material - the first of the year for this box.
She doesn't want to drop it!
Something must be in the air today - the scent of Spring perhaps? Here is a female doing a nesting wriggle in Box1.
And here comes some nesting material for Box1 too!
She drops some of her load on the way into the box . . .
. . . and then gently lays the rest on the floor of the box.
"Well, that's made a start!"