I am not sure what you mean about the stitches stretching. You are going to put your thumb stitches on a stitch holder. (I use scrap yarn because it allows the thumb to bend into its shape.) Cast on your three stitches then knit the next stitches on your left hand needle. IT may seem sloppy the first round or so but it will neaten up. The thumb should be pulled in and rounded.
I am sorry if this didn't answer your question, but I hope it helped.
Lotus Blossom Tank in SWTC Bamboo
Mandarin Blouse in cotton fleece
sockettes in KP Bare tweed
Fat Bottom Bag from SNB - Crochet
Saving Fish from Drowning by Amy Tan