Yes, this would be confusing, as nobody uses it. The German abbreviations are the same as the English ones, as they derive from Latin (dies, hora, pars minuta prima, pars minuta secunda).
So you could abbreviate:
3 d 7 h 30 m
Note that despite such abbrevations being used, they are not as common in German as in English (at least in my experience). In German longer abbrevations such as Std, Min. and Sek. are more common. For Tag, I have seen T. and Tg. If brevity is of the essence, usually a format like 3:07:30 d is employed or less units are used such as 3,3 Tg. or 79,5 h.