How would you calculate Q1,Q2and Q3 in a situation like this ,.1,5,8

When you have LOTS of data, you just count in from both ends 1/4 of the way, and 1/2 way for the Q2, and 1/4 of the way back , for Q3...
but there is another trick:
If you have all the data arranged in order, then :
there will be a minimum m, and a Maximum , M

and then Q1 = (m + Q2) /2
The minimum plus the median , and then divided by two :

AND Q3 = (M + Q2 ) /2

So for your problem , the median is of course 5
and

Q1 = ( 0.1 + 5 ) /2 = 2.55
and Q3 = ( 5 +8) /2 = 6.5