Using the RPi5 with a 3A PS is fine if you don't connect any bus-powered HDDs/SSDs. You can get away using a powered USB hub though.
Other than that I strongly recommend buying the official RPi5 PS (I got white ones about 2 weeks ago) - pretty much all other USB PD powersupplies can't deliver 5A at 5V.
so long,
Hias