This is not
a quick achievement, taking at least 2 months + 1 day, unless you want to throw cash at it (decidedly not recommended, unless you're a multi-millionaire
Every few weeks there is a new event starting, each has 3 champions to unlock, and the further you get into the game the easier they become, so unless you concentrate on them, you might miss some... Just be aware that it can be confusing for new players as it shows them as Year 1, 2 & 3, so make sure you work to unlock each one - you only have to get to area 50, and it gives the option to end it as soon as you get to area 51.
Once you've been playing a while, you'll end up with a base money increase on some areas, plus having a familiar makes this a breeze - once you have both, start pumping levels into the click damage, you can pick up the heroes for the sake of it, but they're not necessary. Generally you'll get to about area 60-70 before you have to have the heroes, so 50 is easier to hit.
As soon as you get 1, move down to the next one, if you're going well each should take about 20-30 minutes - but, once you have all 3 you have to wait for the next event, which is several weeks away. Just as an example about the difficulty, this took me 4 months to get as I thought I could only get 1 per event at the start.
Be patient, and as soon as the new event opens, grab your new champions, and away you go!
Note that new champions only replace existing ones, some have better features, and it depends on your party setup, but you might end up playing with different ones to get the best party for you for longer runs (higher areas). Also, getting the right gear can help a champions become MUCH
stronger, so it might be advantageous to use older champions as they're better equipped...