You need to get a qualified engineer to look at it. It's probably OK if within the outermost 1/4 of the blade but if it is within the innermost 1/3 that is a lot more dangerous.
In general, nicks of this depth can be 'dressed out' with a file; a lot of metal has to be removed because the prop edge has to be restored to a smooth curve to minimise sharp surface transitions which concentrate stress and could cause a crack to propagate. The filed area is then smoothed/polished with emery paper, gradually using a finer grade until it is very smooth.
A 3mm deep nick results in the removal of quite a lot of metal by the time it is done properly, and the prop should be dynamically balanced afterwards.
This is the price one pays from operating from runways/taxiways covered in stones e.g. Elstree.
Eventually the prop ends up below manufacturers limits and has to have new blades put in, or be replaced if fixed pitch - expensive!