The alarms used in homes south of 466A have a 7 year manufacturers warranty. If you are are hearing 3 even beeps the unit has failed, it is not battery problem.
Remove the unit, (twists off), call the number on the back. They will ask for info, date code and description of problem.
If it still covered by their warranty you will receive a new one in 7-10 days at no charge.
I have had three fail so far, all have been replaced as described above, easy-peasy.
Don