Q:

I want them to answer x+3yx+3y and not 3x−2x+4y−y3x-2x+4y-y

A:

In this case, the answer should contain no other variables than xx and yy, and they should each occur only once, and there should be no other symbols than times and plus (or maybe minus), and they should occur only once also.

So create Positive feedback rules and use

"contains variable other than" with definition x;y

"contains variable" with definition x; gt 1

"contains variable" with definition y; gt 1

"contains symbol other than" with definition "times;plus" or "times;plus;minus;unary_minus"

"contains symbol" with definition "[plus,minus];gt 1"

"contains symbol" with definition "times;gt 1" or "times;gt 2" depending on how many multiplications are allowed

(gt stands for 'greater than')