Can iPhone 3GS be used for hotspot & tethering for an Android Tablet?

Yes, provided that your carrier has the hotspot option available.

Kurtosis12 wrote:

Yes, provided that your carrier has the hotspot option available.

And the carrier still has coverage for UMTS/HSDPA (850, 1900, 2100 MHz) and/or GSM/EDGE (850, 900, 1800, 1900 MHz).

True.