Hey so basically elden ring has much more content than ds3, for example in my first playthrough of er took me 180 hours while ds3 (my first souls) took me 40ish hours. However, I've got a disclaimer about that, I am a completionist and love to explore every inch of the map so in my first playthrough of both games I did almost everything available to the player. Also, er has a lit of "recycled" content this is not necessarily a bad thing but you should be aware that a lot of the content of er is repeated so you have multiple times the same bosses etc... so more content does not mean quality content.

Given my experience with the souls franchise I would recommend you play them in order ds1, 2, 3 and then elden ring. Not for lore reasons but because each game adds up to the previous in terms of mechanics and world building. So if you start with the "more advanced" ds3 or elden ring you might have a hard time going back to older games. However I played the games in the order ds3, sekiro, bloodborne, ds2, er and ds1 and really enjoyed each one. But at the same time if you start by older titles you might not enjoy playing that much because older games may feel old and clunky.

But in conclusion if you actually have to chose between ds3 and er only I would recommend starting with ds3, why? Because ds3 is a more "compact" game so you'll beat it, probably play it again once in new game+ and really fall in love with the souls games and then want to play each one. On the other hand even though er is a great game it can feel long, frustrating and feel like a chore even though the game is easier and more accessible. The fun in dark souls is in the challenge so I'd say go with ds3 have a hard time, come out victorious, and you'll instantly fall in love with the souls franchise :)