Alice,
L3d is a subclade of L3. In other words it's an "offshot" with L3 being the parent. Here is what 23andMe has to say about the L3d group:
"L3d appears to have originated in western Africa about 30,000 years ago. Like many haplogroups found in that part of Africa, it spread widely with the migrations of Bantu-speaking people that began about 4,000 years ago. L3d is now found at levels of 27% among the Bantu-speaking Sukuma of Tanzania, and at 10% in Mozambique. Due to the importance of western Africa in the Atlantic slave trade, it is also found among African Americans."

I think you may have received more information from 23andMe based on the money spent especially if you are not sure which surnames to research.

FTDNA does not have a large database of African ancestored people.
Linda