Q: can i use my phones 3g to tether to my wifi only ipad air?

Yes, that can be done. Make sure that your cellular plan allows this

iOS: Understanding Personal Hotspot

http://support.apple.com/kb/HT4517

Use Bluetooth to tether your iPhone, iPod touch, or iPad

http://www.macworld.com/article/1159258/bluetooth_tethering.html

How to Connect an iPad to an iPhone Via Bluetooth Tethering

http://techtips.salon.com/connect-ipad-iphone-via-bluetooth-tethering-25472.html

 Cheers, Tom

I have been able to do it using joiku spot on Nokia E72 that uses the data connection and use it on iPod Touch. No Bluetooth needed.