Technically, the three seasons up till DxD Born have covered 6-7 LNs. BUT the thing is, they changed the whole LN events placement in Born and basically skipped the main events of the 5th LN, the battle with the Sitri peerage. They also changed the whole plot of the 7th LN too. Not giving any spoilers but its a pretty awesome part of DxD's plot as a whole. I recommend you watch the first two seasons and start reading the LNs from Volume 5 on wards, that's what I did, 3 years ago (Note to self- )
Tbh, DxD Born felt poorly planned and kinda disappointing, if you've read the Light Novels.
1st season--> LN 1,2
2nd Season--> LN 3,4
3rd Season--> LN 5-6-7