That's illegal - you can't advertise a price (such as the conference cost) but have no way that people can pay that price without also being charged an unavoidable fee.
If a surcharge is applied to specific kinds of payment, there needs to be at least one payment method that attracts no surcharge: otherwise the total price (including the surcharge) needs to be the advertised price (similar to how Aussie airlines have to advertise fares inclusive of all taxes, or how prices quoted to the public must include GST, as these are unavoidable charges).
Depending on the size of the conference company, a 3% surcharge on Visa & MC may also be excessive and in breach of the new RBA regs which came in last year for larger businesses, or which apply to all business from later this year.