Can.. no one will stop if you go for a 3-way system.
Just that as 3-ways are more difficult to tune and get right, you may lose interest and get fed up quickly. Also there are more things to conside when doigna 3-way such as mounitng and speaker placement is more critical, things which you wont fully understand yet.
However, you can buy a 3-way system and use it as a 2-way first. Once you understand what you want to achieve and some basics on tuning, you can easily install the 4" midrange.