An = a + (n-1)d 
where an= nth number 
a= 1st number 
n= number of integer 
d= common difference 

Let a=x 
A.P. will be: x, x+d,x+2d_ _ _ _ _ _ xn 

The product of the A.P. is 1 
therefore the A.P. has 1 in it or half the numbers in A.P. are in fraction and the rest are whole numbers which cut them off.... 
Like 1/2 x 2=1 

Then the A.P. must be 
x. x+d, x+2d_ _ _ _ _1/x ( as xn will be the reciprocal of x such that the product becomes 1)
1 5 1