I wasn't planning on it... she's staying in the craftroom with the
door CLOSED for at least a week. Maybe even longer. We'll go in and
play with her and come out and try to play with the girls and see if
they get used to each other's smells that way and quit hissing.

I just kittenproofed the craftroom and swept the floor and plugged the
feliway diffuser into the wall. Should it be in the craftroom with the
kitten or out in the livingroom?

Many integration purrs!  As I said earlier, Silver and Scout will be
upset at first, just as children are upset when a new brother/sister
arrives or the sort of upset/stress you could expect if a friend moved
into your house to stay long-term.  Over time, Tessie will "belong"
there and then Silver and Scout will get agitated if she is ever
absent.

(I remember, after two months of loudly expressing her hatred of
Smokey's guts, we took Smokey to the vet for his neutering and Nocturne
was walking around crying, pleading with us...she was lonely.  She
wanted Smokey back.  Once he returned, she sniffed him a few
times...and an hour later was back hissing at him when he got too close
;)

Also, when we went to get them at the kennel this year, we saw them
meatloafed on a shelf, facing one another, FAR CLOSER than they ever
sit in the apartment, even though the kennel had two rooms and multiple
shelves.  That lasted for an entire week once we got them home ;)

--Fil