That could take a long time!
There is the whole family of a/10 - b/10 where a = b + 3
Eg 4/10 - 1/10, 5/10 - 2/10, 6/10 - 3/10 etc
Or you could get sneaky with 5/6 - 8/15, 27/40 - 3/8, 1/2 - 1/5, etc
There are an infinite number of fraction pairs with a difference of 3/10
Start with a multiple of 10 on the denominator eg 50 =5 x 10
Now pick numerators with a difference of 5 x 3 eg 11 and 26
This will give you 26/50 - 11/50 = 13/25 - 11/50 = 3/10
You will get more fun problems if you avoid relatively prime fractions so they will simplify
Eg denominator 60, numerator difference 18.
59/60 - 41/60 = 3/10 is not nearly as much fun as 45/60 - 27/60 = 3/4 - 9/20 = 3/10