Q:

How do I roll when a weapon has multiple different damage types?

So I've been out of 5e for years and I'm recently getting back into it and I had a question about one of my weapons. It's a sword that does 2d6 piercing and 1d6 force. Do just roll 3d6, or do I do separate rolls?

A:

Mostly 3d6
The large majority of monsters do not care about force damage either way, and the sword must be magical if it has this special power.
Few exceptions
Against monsters that have resistance or vulnerability to force, but not slashing1, (or the other way around), you have to roll separately.
Swarms for example resist slashing damage even from magical sources:

1d6 is not halved
2d6 (plus your Str or Dex modifier, and whatever else you add) is halved

Solution
Always roll the force 1d6 with a dice of a different color, but tell your DM the total result, and mention that X of it is force, while Y is magic slashing.
As stated above, most of the time he will not care, and you do not want to make his job unnecessarily harder. If he cares, he will let you know.

1) Or piercing, or whatever kind of damage the base sword does.

