（1）
m²/(m-n)+n²/(n-m)
=m²/(m-n)-n²/(m-n)
=(m²-n²)/(m-n)
=(m+n)(m-n)/(m-n)
=m+n
(2)
(x-3)/(1-x)-(1-x)/(x-1)
=(x-3)/(1-x)+1
=(x-3+1-x)/(1-x)
=-2/(1-x)
(3)
1/(1-a)-1/(a-1)+3a/(a-1)
=(1+1-3a)/(1-a)
=(3a-2)/(a-1)
题目写得很混乱,建议你多用括号,表达清楚些,再追问,我再给予解答.