There are a number of things you write that don't line up.
python3 -m pdb <sourcepath> <args>
invokes pdb, not ipdb. realgud looks for a prompt, and the one for ipdb is ipdb>. The prompt for pdb is (Pdb) and will not trigger look for postiion informatation when the wrong kind of prompt is printed.
Finally, I should say that if your are ...