Why can't I add to a float?

Im not able to add small numbers to my float.

For example, ive got a float thats set to 10,000,000

If I add 3 to it, nothing changes. If I add 3,000,000 it does.

What do I need to do to make sure this kinda thing doesnt happen?

This has to do with the (lack of) accuracy of floats.
I won’t go into the details here, but floats are generally accurate withing 7 digits.

10,000,000 is 8 digits long, and adding 3 only affects the last digit, which is outside the accuracy range. Adding 3,000,000 affects digits well withing the 7 digit accuracy range on 10,000,000.