We moved to New Zealand in 2012 renting out our house in Colorado from September 2012 until the present time. We want to sell it and heard that if we sell it within three years of renting it, it is still considered our main residence and we will not have huge taxes to pay. Is this right or will selling it now result in huge tax consequences and if so how much? It is probably worth around 850,000. We do not own any other property.
You can exclude up to $250000 ($500000 for married filing jointly) of capital gains on property which was your primary residence for at least 2 years within the 5 years preceding the sale. This is called "Section 121 exclusion".
See the IRS publication 523 for more details.
Gains is the difference between your cost basis (money you paid for the property) and the proceeds (money you got when you sold it). Note that the amounts you deducted for depreciation (or were allowed to deduct during the period the condo was a rental, even if you chose not to) will be taxed at a special rate of 25% - this is called "depreciation recapture", and is discussed in the IRS publication 544.