more sonar sensors

[phol33]

Hi everybody!
I am a complete rookey in Lego NXT and I want to build a robot which has 3 sonar sensors and I only heard about some problems with using more then one sonar sensor.
I heard that only port one can handle a sonar sensor so what do I have to do to you use 3 Lego's sonar sensors? Do I have to rewrite the I2C Protocol?

I hope someone can help me!

THX

Philipp