My daughter holds her poo in also 3.5 years old , it causes absolute nightmares when it gets really bad , I started putting a small dose of lactulose in her evening drink (milk / juice ) took a while but now she can pass it easier, we still have the odd battle here and there , but I found it really helped us x
The lactulose makes the urge stronger to go to the toilet so she has to let it go , because she isn't constipated she's holding it in because shes scared , so we found this the best way to go , hope things get better for u
We had the same issue about a year ago when my daughter had started potty training. The GP prescribed Movicol to help regulate bowel movements x