It requires quite a bit of DM intuitiveness, but overall it's not super hard. If you're familiar with the edition, you can get a much better conversion, since you'll understand the feel of the original better. Ability checks and saving throws should just fall under the usual easy/average/hard type, but when dealing with 3E you have to remember that higher DCs were meant to challenge those who were skilled (take Prof modifier into consideration). I'd redesign each combat based on the 5E encounter design and CR system, but often in earlier editions some combats were deliberately "unfair," meant to be avoided or handled more tactically. Treasure has honestly been the biggest PITA, since earlier editions had completely different economic assumptions, especially when it comes to magic items.