Lots of ways:
1--start with 3, increase every row, (work in garter) continue till shawl is desired size, bind off
(1 A to 1 Z--add borders, work in stocking knit, add lace, add fringe, (etc))
2--start with 3, Increase on one edge (every row, or every other row) when shawl is 1/2 desired size, switch, and decrease every row, (on same edge that you had previously been increasing)when you have 3, bind off.
(variations 2A to 2Z--add border, add lace, work in stocking knit, add fringe, etc)
3--double V from neck--
Cast on 7,
Mark center stitch.
for first 5 rows, increase either side of center (2 increases every other row) (move marker down as you work)
Place marker after stitch 5, (leave marker on center stitch) Place marker Before last 5 stitches.
Increase 4 times every other row.
Once at first marker, one each side of center (2) once again before last 5 stitches.
Continue till shawl is large enough. bind off
(variations 3A to 3Z? Keep border in garter, make stitches after the first, (and before the last) markers stocking knit or lace, add a border on lower edge, change border from garter to lace, etc)
Cast on between 600 and 1000 stitches
(or make a border --at least 600 row, up to 1000 rows in border then pick up 600 to 1000 stitches from border edge)
Mark center of your stitch count
Knit, (making a border at first few stitches--if desired)
Every other row, make a raised center decrease at center point (marked center)
continue till you have 3 (to 5) stitches. bind those off, and make a small seam at center back (of neck)
(variation, decrease 4 times every row, (at borders edge, center back, and before border starts)
REALITY? check out this SITE: 352 simple triangular shawls
the patterns are sorted by yardage, and will give you tips on yarn, yardage, needle size, and finished size of shawl (also may have photo's) NOT ALL ARE FREE--but many, many are!
Still you'll find, MOST Of the designs are variation of the 4
basic designs i wrote above:
side to side
top down double V
some have borders, some have lace, some have fringe, (etc)
but in the end, these 4 basic shapes cover MOST (not all!) triangular shawl patterns.
(other, "V" shapes (really a V when laid flat is not a triangle, but a V(there is a small (to large) notch for neck--)
Piecework (think of knit interesting squares (not granny squares of crochet, but something like a granny square--only knit)some times the square are triangles, or other shapes.
A flat knit V --all the above V are 'half a square" (right triangles) but you can knit other triangles.. think of equalateral triangle (all the side are the same size)
start in center, cast on 6, every row (and some times more than every row!) increase 6 times, in 2 pairs, to make a center out triangle.. (but this is getting away from easy!)