What can happen if I have not filed taxes for 3 years and the IRS is sending certified letters and I do not answer them?

The IRS can estimate your taxes, file a return based on their estimate and start collection activity for the estimated amount (plus interest and penalties). Collection activity includes seizing your bank account, garnishing your wages and, at the extreme, selling your house and taking the proceeds. File your tax returns, even if you can't pay the tax.

They can take your assets and garnish your earnings.

NOTHING GOOD. They will eventually come and get you and /or your property and accounts.

The IRS may assess an estimate of the taxes owed for each year. These estimates are almost always remarkably high. The IRS may then place a lien on any property you own and eventually garnish bank accounts and wages. Interest and penalties that accrue while no agreement are in place are approximately 4 times what the interest rate is on an agreed payment plan. It is in your best interest to file taxes and resolve issues with the IRS. Once a lien is placed, interest and penalties on the original tax may not even be dischargable in bankruptcy.

Since you did not file the required tax returns, the certified letters from IRS may be to let you know that IRS will be assessing taxes against you based on the W-2 and/or 1099 information they have from your employer(s). You should review the letters and respond by filing the correct tax return.