Not all; Atalanta was also on crew of the Argo, and isn't shown.
Along with about eighty others, at least a quarter of whom are well known enough to end up having Servants based on them. Damn boat was basically a "who's who" guide of Greek legendary heroes of the pre-Trojan generation.
it'll be a quartet if Atalanta was also drawn in this pic. XD
Yes, but given that she IS already a known, that makes this An Argonaut Trio, not the Argonaut Trio, since any combination of two of these and her would equally fit.