this is not a difficult problem at all
Seed stitch=(right side)K1,P1
(wrong side) knit purl stitches, purl knit stitches
Stockinette stitch=(right side) knit
(wrong side) purl
twice= two times
here's the long way to write those instructions:
work in seed stitch across to next marker, work in stockinette stitch across to next marker, work in seed stitch across to next marker, work in stockinette stitch across to next marker, work in seed stitch across
"twice" means you do what is written in parentheses two times.
it sounds like there are 4 markers on the needle (not 3)
There, if there are any more problems just post the confusing pattern instructions in a thread, there is a whole website full of people who want to help.
If I had to visualize this blanket, I would assume that it has a border (probably of seed stitch) and then it has large blocks of stockinette stitch seperated by seed stitch. It's just a rectangle with squares of different pattern stitches, right?