ECDH for more than two parties
You can do ECDH with more than two parties. See the below adaptation of the wikipedia example for an EC group -


The parties agree on the algorithm parameters, a curve over $E(\mathbb{F}_p)$ and base point $G$.
The parties generate their private keys, named $a$, $b$, and $c$ (these are integers).
Alice computes $aG$ and sends it to Bob.
Bob computes $(aG)b = (ab)G$ and sends it to Carol.
Carol computes $(abG)c = (abc)G$ and uses the x coordinate as her secret.
Bob computes $bG$ and sends it to Carol.
Carol computes $(bG)c = (bc)G$ and sends it to Alice.
Alice computes $(bcG)a = (bca)G = (abc)G$ and uses the x coordinate as her secret.
Carol computes $cG$ and sends it to Alice.
Alice computes $(cG)a = (ca)G$ and sends it to Bob.
Bob computes $(caG)b = (cab)G = (abc)G$ and uses the x coordinate as his secret.


A note about your concern with Montgomery curves (such as curve25519). Montgomery curves indeed only return the x coordinate but point multiplication only requires the x coordinate as input. Said another way, the Montgomery ladder which is used for point multiplication does not use the y coordinate in the algorithm. So the doing e.g. $(aG)b$ is fine since $aG$ will return an x coordinate, and we can use the Montgomery ladder to multiply that by $b$.

In fact, multi party DH will work for any group that has a "combine" and a "scale" operations for its elements.