I am trying to separate a decimal number by digits.
for example
from the number 12.34 to get the first digit we can multiply (12.34100)/1000.
But Arduino is not letting me do ((12.34100)/1000)%1000

How to do this on a float data type to get the 3rd digit and the decimal digits..?

Decide how many digits right of the decimal point you want. Multiply by a multiple of 10 to move that number of digits left of the decimal point. Cast the result to int, and use % to extract the individual digits.

So, if you want 3 digits after the decimal point, multiply by 1000.0 before casting the result to int. 1.234 then becomes 1234.

nemo4all:
Ahh…
My mistake was… I should have done ((12.34*100)/100)%10

You’ll still have the same problem unless you explicitly force a conversion of the intermediate result to an integer data type (as already suggested).
Multiplying a float by 100 and dividing the result by 100 will not achieve much.