re: I've been given $100K from an estate - Pay down mortgage, get new car, other?

You could potentially have to pay a small amount of tax on this, if there was a passage of time between the death and the time you were placed in possession (i.e. given the money). The executor could send you a K-1 for the taxable income earned by the estate while it was open. It would be pro-rata based on the distributions. Again, this would be based on the taxable income of the estate, not the amount you were given.

Any income allocation may be very small and would be due with your 2016 taxes, so I would call the executor and ask if this is a possibility.

Beyond that, if your kids are only three, see if you can find a sitter for a few days and go on a nice trip with your spouse, toasting and praising the deceased.

Beyond that, I'd max out retirement, put 25K in college savings for each kid, beef up emergency fund, and anything left, I'd put into a retail index fund.