Perhaps to clarify for the original poster....
There are only a few AEGs that come with 3rnd burst (i.e. Systema PTW, Type 89). AEGs can be modified with a computerized mosfet to enable 3rnd burst...but typically they all have semi and full auto [generalization]
As it pertains to airsoft M4/M16's...they'll likely be semi/FA.
As said previously...it's a fair bit of work and expense to convert one to the other. Sometimes worth it (time/cost)...but it is far easier to simply buy the other model outright.