All three statements are grammatically correct, and could theoretically be uttered by someone. If the statement is a public announcement, however, the most likely version is
Firefighters will test hydrants ...
The rest of the statement, being a public announcement, would likely have specific information such as
...between 8 A.M. and 6 P.M. Monday through Friday
Other similar announcements are
The performance will last three hours
Flight 7432 will depart from Gate B 44
The candidates will meet for another debate in three weeks
A more general public announcement about the hydrant-testing would probably read
Firefighters will be testing hydrants around town next week. Users should expect some fluctuations in their water flow.
In conversational style, one would use either
Firefighters will be testing.... OR, more idiomatically,
Firefighters are testing hydrants around town next week. Do you want to go and watch some of them?
A good rule to follow is to avoid will + base form for a purely future idea in everyday conversation, especially in first and second persons, but also in third person. This form has many other interpretations that can lead to misunderstandings. One very common question that teachers hear from their non-native speaker students is
(To a teacher) Will you give us a quiz tomorrow?
...when the student is not requesting a quiz but merely inquiring about the teacher's plans. The non-request question, if the speaker wants to use will, should be
Will you be giving us a quiz tomorrow?