It is totally reasonable to communicate to the professors that you are considering working with others in the Department. I would further argue that being able to communicate transparently about difficult things is a good signal to faculty that you might be good at communicating with your supervisor generally. Because these professors know one another, probably collaborate together, and would have probably developed relationships, I would assume there is a level of comradery among the faculty. If there isn't, this would indicate to me a potentially unhealthy work environment for your graduate studies.
Consider then the following couple of cases:
1) You accept the first offer that comes your way from professor A
In this case you secure a supervisor and your path forward, for the moment, is clear. The opportunity cost of accepting the first offer that comes your way is that you may potentially lose the offers from the other professors with whom you're more interested in working with anyways. The reason for the lost offers could range from professors B and C not wanting to try and "steal" you away from their colleague or perhaps professors B and C could also each accept a student in the time it takes you to decide; there are many ways that situation could evolve. Whatever the case may be you run the risk of restricting your options right away. An additional risk is that you might accept the offer and then find that you'd rather work with another prof once they give you an offer letter, which puts you in the position of now having to retract your acceptance from prof A and for some people this could be really hard to do.
2) You hold off until all of your offers come in
In this case by waiting out for the offers from B and C, you run the risk of potentially losing supervision from A if a student more suitable to their interests and research needs comes their way. The benefit, however, is that you might secure a spot with a professor who you are more personally invested in working with. If you think that an offer letter from B and C is likely, then you should weight the risks according to your risk tolerance.
At the end of the day what is the most important thing for a graduate student is having a supervisor that you are able to tolerate (but ideally enjoy working with), a supervisor that is invested in your growth as an academic and individual, and a project you can find some excitement in for a long period of time. Working with a supervisor that you don't really want to work with for 1.5-2.5 years (masters) to around 4-5 years (PhD) is a slow hell that you probably have been exposed to on this forum and is a scary reality for many people.
Just because you were given an offer letter right away does not mean you have to accept it right away. Expedience can sometimes be the death of long term happiness, and in graduate school (particularly a PhD) you want to think what will give you the most joy (academically, professionally, and personally) on average over the course of a 4-5 year basis.