There, in fact, is:

According to Collins:

Prof.
Also prof.
Word forms: plural Profs

However, I would prefer using Professors rather than Profs.

Here are two (random) examples of Profs. being used:

Dear Profs. Lee and Winn,

Thank you both for your valuable comments on the manuscript. We greatly appreciate your time and consideration, which has helped us to vastly improve the manuscript.

  [...]

(Source)

Dear Profs. Garcia-Olmo, Strom, and Tarnawski,

We would like to thank you and the reviewers for the positive remarks concerning our article

  [...]

(Source [Download required])

Is there an abbreviation for "Professors" plural, in a greeting at the top of a formal letter?

Yes, according to Collins:

Prof.
Also prof.
Word forms: plural Profs

The plural form of Prof. (Professor) is Profs. (Professors).

Here are two (random) examples of Profs. being used:

Dear Profs. Lee and Winn,

Thank you both for your valuable comments on the manuscript. We greatly appreciate your time and consideration, which has helped us to vastly improve the manuscript. [...]

(Source)

Dear Profs. Garcia-Olmo, Strom, and Tarnawski,

We would like to thank you and the reviewers for the positive remarks concerning our article [...]

(Source [Download required])

There, in fact, is:

According to Collins:

Prof.
Also prof.
Word forms: plural Profs

However, I would prefer using Professors rather than Profs.

There, in fact, is:

According to Collins:

Prof.
Also prof.
Word forms: plural Profs

However, I would prefer using Professors rather than Profs.

Here are two (random) examples of Profs. being used:

Dear Profs. Lee and Winn,

Thank you both for your valuable comments on the manuscript. We greatly appreciate your time and consideration, which has helped us to vastly improve the manuscript.

[...]

(Source)

Dear Profs. Garcia-Olmo, Strom, and Tarnawski,

We would like to thank you and the reviewers for the positive remarks concerning our article

[...]

(Source [Download required])