Compare your old tubes to see if the punctures are in the same place. If so, then it’s something sticking in it, either rim or foreign object in the tyre.
Aligning tyre labels to valves etc sounds anal but it can help in showing where a puncturing object may be in the tyre.
From your description of 2-3mm, I’d suspect pinch flats, especially if you are using a hand pump. They will be generally on the inside edge of the tube, in line with the rim.
I run 35mm Marathon Greenguards at max pressure of 85 psi on my commuter. 25 miles each way on/off road, fully laden with panniers etc, and not a puncture in sight. Previously used Michelin Tracker, same result, but eventually split the tyre after 3 years abuse.
Marathons are tough, a bit slow and lifeless in 35mm flavour.
But as above, get some pressure in there on your current tyres!