Basic Rules for 5eThere's a lot of changes between 3e and 5e. That being said, if you're familiar with the math of 3e, it shouldn't be too difficult to learn 5e. It doesn't really have anything weird like negative AC like in ADnD.
Here's a couple notable differences that I can think of:
AC works slightly differently as you can no longer target a lower value if you're using touch attacks or your target is flatfooted. In 5e AC (alongside attack bonuses and values in general) tends to scale much lower than 3e due to the concept of bounded accuracy.
Proficiency replaces BAB and is used in multiple different mechanics such as skills and saving throws. Your proficiency bonus is a value that scales with character level (not class levels) and goes from +2 to +6. You don't gain any iterative attacks from this bonus like you would from BAB. Instead you gain more attacks from the Extra Attack class feature. On the flip side, extra attacks no longer suffer a stacking -5 penalty (they're all made at the same attack bonus) and you can move before, between, and after making your attacks.
As touch AC no longer exists, touch attacks (and ranged touch attacks) don't either. Instead these spells now work as Spell Attacks which are calculated with Spellcasting Ability Modifier (INT/WIS/CHA) + Proficiency vs target AC.
Spell Save DCs are calculated differently than in 3e. Instead of being 10 + spell level + casting ability it's now 8 + proficiency + casting ability. There's 6 types of saving throws now (one for each ability score) instead of 3.
Casters get access to cantrips. These are at-will spells that can be used an unlimited number of times and scale with character level.
Skills work very differently from 3e. You no longer put ranks into a skill but instead choose a number of skills to be proficient at (you add your PB to the roll alongside the relevant ability score modifier). You can increase this value even further if you have expertise in that skill, which you can gain from the expertise class feature or the skill expert feat.
Feats are not nearly as common anymore. Instead you now have to choose between Ability Score increases and feats. Most of your power increases comes from your class features rather than feats.
