Privateer Press has since stopped releasing expansion books, and newly designed models/units no longer come with physical cards. Older models/units that are still new-in-the-box still come with cards, but those cards are (probably) outdated since Mk3 included rebalancing across almost all models. In order to get back in the game, there are two official options:
- War Room 2 app
- The card database
The War Room 2 app is free to download, however you must buy faction decks in order to unlock the cards for that faction. The app is great for list building, and you can start a battle with a list which allows you to track damage to models, how many models are left in a unit, and other useful stuff.
The card database is great for players, like myself, who prefer to use physical cards at the table. Each card is guaranteed to have the latest updates in the database. You can select which cards you want to print, and how many copies, and the site will create a PDF with 4 cards per page for you to download. You can then print them out yourself, cut them out, and put them into card sleeves like you used to.
Privateer Press has gone this route because the game is now developed and balanced through their Community Integrated Development (CID) process. This allows the community to actively play new and existing models that are still in the final stages of development and/or (re)balancing. Any revisions to rules for existing models will include updates to their cards, which is why it doesn't make a whole lot of sense for Privateer Press to include cards with new models.
There's also an unofficial source for model rules, and that is Warmachine University. It's a wiki and the community that updates it does a pretty good job of listing all of the rules of a model/unit, and includes plenty of commentary on the effectiveness, weaknesses, and synergies. I find it a great place to get an initial impression on models I'm not really familiar with.
Another thing that is a different with Warmachine in Mk3 is the fact that you can now pre-measure anything at any time. With this change, there are now different products available for measuring sticks that range between half-inch all the way through 14 inches. As long as you have a good tape measure, you don't need measuring sticks... but it certainly makes it easier to measure threat ranges. This was the most surprising change for me when I got back into the game myself.