I think He was talking about His death, and of course the apostles were alos martyred. He certainly did not mean water baptism.
This passage raises questions. We’re the 12 Baptized at all (other than John’s Baptism)?
Jesus may have referred to their Baptism at the washing of their feet.
Then he poured water into a basin, and began to wash the disciples’ feet, and to wipe them with the towel with which he was girded.
6He came to Simon Peter; and Peter said to him, “Lord, do you wash my feet?”
7Jesus answered him, “What I am doing you do not know now, but afterward you will understand.”
8Peter said to him, “You shall never wash my feet.” Jesus answered him, “If I do not wash you, you have no part in me.”9
Simon Peter said to him, “Lord, not my feet only but also my hands and my head!”10Jesus said to him, “
He who has bathed does not need to wash, except for his feet, but he is clean all over; and you are clean, but not all of you.”
11For he knew who was to betray him; that was why he said, “You are not all clean.”
Also, I’m not sure that John was martyred. And is that what you interpret as “my Baptism” or “the Cup”?
I think that Jesus tries to say they are unable to accomplish what these require, but after Jesus accomplishes them, He will freely give them through Baptism and His Eucharist.
They had belief to receive, but not belief to accomplish the work which God demanded of the messiah. But through the work of the messiah, all who believe are freely offered His merits.
So Jesus asks if they are able, but then says yes you will receive because I have done the work for you. But to each is given their own crosses to bear. And continuing in receiving His Eucharist means doing so with a pure conscience. So that what we do in Him is acceptable to God, and of course so we are able to do good works in Him.
Jesus also gives them the Spirit by breathing on them. This is for the Sacrament of Reconciliation. This may be what Christ’s Baptism was. When Jesus received Baptism from John, He accepted sin on Himself, since He was without sin. These are my thoughts.