Normally there is an extra person charge,usually starting with 3rd adult, included in view details of your rate when you check the booking.

If there is this extra charge then the rollaway should be free.

We just had suite stays at the Singapore Intercontinental and Crowne Plaza Changi, no sofa beds, and were charged $100 and $80 for the 3rd adult (12yr old son) this was in rate details.
Before we left home I requested a rollaway for our son and I.C. Said all good but Crowne Plaza wanted to charge $80 for it. When we arrived at hotel I showed booking confirmation with extra person charge and they waived the extra fee they wanted to charge for the rollaway at Changi.