D or 3 for Transmission life extension?

Hello everyone. I read somewhere on some website that driving short distances or around town or up to get the groceries it's best to drive in 3 than in D, to put less of a load on the transmission. is this true? I cant really notice very much of a difference between driving in the two, but im a beginning driver. but i do notice that in 3 it drives slower, needs a bit more gas to move more than while in D, but not sure. Only got 36k miles on my 96 svx, and i wanna try me darndest to keep the transmission alive and kicking. what should i do about driving in D or 3?