Find 3 rational numbers between

add the two numbers and divide the addition by 2,

 so you get -1/10

so we have two three numbers

-3/5, -1/10, -2/5

now take the two numbers -3/5, -1/10 , take their addition , divide it by 2, that is the next number reqd

 similarly,

take -1/10, -2/5 add them divide the addition by 2 , that will give you the next required number.