Cast on 3 stitches
Knit 3
Yarn over, knit 3
Yarn over, knit 4
Yarn over, knit 5
and so on. Of course, by increasing 1 stitch at the beginning of each
row (not the second stitch!) all the increases are a little loose. The
pattern I've been following is actually for a raised leaf and lace afghan,
but it starts in garter stitch. It could be done in any flat stitch, including
seed stitch or alternating rows of knit 3 pearl 3, as long as the edges
stay flat.
I believe a good pattern would be:
Cast on 3
knit 3
yarn over, knit 3
yarn over, knit 4
yarn over, knit 5
yarn over, knit 6
yarn over, knit 3 pearl 1 knit 3
yarn over, knit across
yarn over, knit 3, pearl 3, knit 3
and so on, creating flat edges of garter stitch with a stockinet body. When the triangle was large enough - depending on yarn and gauge - I'd end with three or more rows of garter stitch.