I made a substantial donation to my sponsoring rabbi's discretionary fund and a "several times chai"-size donation to the discretionary fund of the associate rabbi (he was leaving in a month to lead a congregation of his own, so I asked him if he would prefer to have a donation made to start out his discretionary fund at his new shul, but he said that he could use it at the current shul) and the same sized donation to the general fund of the summer camp directed by the third rabbi. If a rabbi does not charge a fee for services related to conversion (some do), then I think that a donation to the rabbi's discretionary fund is the best way to show appreciation.
Back when we converted our children, my husband gave honoraria to the two rabbis we didn't know (perhaps a larger amount to the head of the Beit Din), but we did personally know the third rabbi as a knew him as a regular member of our lay-led minyan. He is the same summer camp director who was on the Beit Din for my conversion. He seemed almost hurt that compensation would be suggested. So I think my husband made a donation of the same amount that he paid to the other rabbis to his summer camp, which he appreciated. In addition to being the camp director for over 30 years, he has always personally been a big donor to the camp, so he always appreciates any support for that cause.
In the case of my sponsoring rabbi, my family had already been paying a full family membership to his shul for several years so that our children could attend the Hebrew school (after the shul where they started Hebrew school merged with his shul). We were unaware that as a non-Jew, I could not have any type of "membership" status, so we were really supposed to pay only a the fees for single adult plus two kids. And I guess this did show up in the membership forms, even though at least some people in the Hebrew school knew I was not Jewish. (But the departments do not necessarily share information.) That essentially amounted to a rather sizable "unintentional donation" of many thousands of dollars over the years. So I guess I could have rationalized that I'd already more than "paid" for my share of a the rabbi's time.
However, I appreciated that my rabbi accommodated the fact that I could not attend the conversion classes due to a time conflict with my children's scout meetings and instead made time in his very busy schedule for many one-on-one sessions with me. I felt that was way beyond the services required by a typical congregant (even those who might need him for times such as a death in the family). When I first started to meet with him, I had asked him how I should compensate him for his time and he said that he has never charged for working with conversion candidates, but that I could make a donation to his discretionary fund when my conversion was complete.
I thought of all the money I have paid for tutoring and music and sports lessons for my kids and felt that the time my rabbi spent with me was worth at least that much. Additionally having been recently blessed with a new job (that was a new position in the department which suited me perfectly), I felt that I could well afford a good-sized donation.
In addition, I wrote personal letters to thank each of the three rabbis. In the case of my sponsoring rabbi, it was a letter of several pages. I did know each of the three rabbis personally, so I was able to thank them specifically. This is something that costs next to nothing and may be a better way to express your thanks in a meaningful way. I happen to feel that people are thanked less than they ought to be and that people are quicker to complain than to praise, so I have always tried to wrote special thank you notes to let people know when they are appreciated such as at the end of the year for each of my children's elementary school teachers.
I think you should take into account how much time and effort was required on the part of the rabbis. If it is a local thing and the other two rabbis just show up for an hour, they don't need as much acknowledgment compared to a rabbi who spent a lot of individual time with you or a rabbi who had to travel a great distance as when there is no locally available mikveh.