The "Open range 3-max" tool will help you in both tournaments and cash games. When using it for cash games, don't forget to set the ante number to 0 (none). You will sometimes have 3 on the cash game tables when you try to start new tables of shorthanded (tables with 6 seats). For example, you see that there is no action at your limit in your favorite poker room (or all the tables are already full) and therefore you sit down at a table alone. A player joins you (in which case you will need this tool) : https://www.push-or-fold.com/open-range-2max.php) and a little later, a third player will join you
This tool is also useful for the increasingly popular spin-type sit'n'gos at most online poker rooms. In a nutshell, a spin sit'n'go is a 3-player hyper turbo sit'n'go where the prize money is drawn at random between 2x and 1000x the starting buy-in (numbers may vary by poker room).
This tool is very intuitive. You indicate your position, SB or BTN (the BB position will not be able to open, which explains its absence), the number of antes (0%, 10%, 12.5%, 20%) and your number of blinds (between 20 and 100). Clicking on "Can I open" afterwards will give you the table that matches your situation. The hands are divided into 6 categories. For a detailed explanation of the colors, you can read this article : https://www.push-or-fold.com/open-range-2max.php
Ranges are GTO (Game Theory Optimal) ranges, i.e., preflop opening ranges that are profitable in the long run against anyone (it's up to you to make the hand profitable postflop). On the other hand, as you get more information about your opponents, you can adjust the ranges. To take two extreme examples, if your opponents never fold to raises, avoid playing hands categorized as marginal opens. On the other hand, against very conservative players, feel free to play all hands in gray (marginal opens). These opens are EV $0 in the long run against all players, but will be slight losers against the sharks and winners against the fish. Think of these charts as a guide or compass, but also rely on your instincts, your notes, your experience and the dynamics of the table. By "table dynamics" we mean the impact that previous hands will have on the players at your table. For example, be aware that a player who has just had a terrible badbeat the previous hand, will probably be less likely to fold versus a 3-bet or 4-bet bluff. Also, if you are in a "card rush" (a successive series of very strong hands), tend to play your future big hands very aggressively. If you have made several 3-bets in the last 5 hands and the chart tells you to call with AA rather than 4-bet, in this situation you may be better off 4-betting since your opponents will start giving you less credit.
That's the beauty of poker, where a mixture of art and science come together