The Diana 48/52 is probably the most awkward gun to load with all 3 levels of safety at once.
What I do is:
Hold the gun on it's side, butt pad tight in the pelvis area.
Pull the cocking lever up/towards me to the first click of the ABT ratchet. That allows plenty of room to load.
Still holding on to the cocking lever, the flat of my left hand can press on the gun just forward of the breech while finger & thumb fit the pellet in.
Then cock it the rest of the way.
The above only uses two points of safety (ratchet + holding on to the cocking lever), but once the lever is cocked all the way (engaging safety point 3 - the trigger sear), it seems impossible to effectively control safety point 2 (holding on to cocking lever).
Plus, if things do go wrong, there's a lot less pressure there at the first ratchet click compared to fully cocked all the way.
The above is an evolution of a HW77 technique, that involves pulling/holding the cocking arm just enough to slip a pellet in with the other hand (thanks to Mick for that one ).
HTH
P.S. In my view, the two major factors in these springer 'accidents' are poor loading techniques & slippery (oily) guns.