Hmmm, do you mean you're attaching the 3.5" piece to the next row's 3.5" piece? Or are you going in your first row - 3 8 3 8 3 and in your second 8 3 8 3 8?
Assuming you're doing the latter - that's an easy problem! Each row has to start with a 3 and end with an 8, or vice versa. By having it go 3 8 3 8 3 = 25", with a 3 on each end, it won't be even with your 8 3 8 3 8 = 30 "row, unless you cut parts of the 8 off. And if you stop at the 3 in the 2nd row, it's too short :(
So as long as each row starts with one size and ends with the other size, they'll line up :mrgreen: Or, if you don't want any seams to line up, just make sure that the rows add up to the same number, after you take out seam allowance (like what I did in the 2nd paragraph)
Happy sewing!