I have found another method, which is a compilation of all the methods I've seen so far.
My goal is never use units to look for other opponents.
I don't use sandbagging anymore.
Battles 1 to 9 -
It should be easy.
Battles 10 to 12 -
I don't use sandbagging.
I just play the lowest 3* teams.
Battles 13 to 19
- I use 3 4* 3/30 From battle 20 and on
I just keep my team total PI over 4200 (with no synergies or boosts).
This is not the fastest method because if you don't have enough teams of 4*s you'll have to wait for their cooldown.
gaming.stackexchange.com/users/113859/ChaoticGabibo
Well, I guess I'll just post what I found on Reddit :
Battles 1 through 8 or 9, easy. 9 through 15, sandbag.
Play 2 champs around the same, and 1 really weak one.
If you get a match up too difficult, pay the credits for a new matchup.
I prefer to use champs that are around 1200 PI each.
16, 17 and 18
, Keep your team PI above 5500.
Typically you need to use 4*s that are 3/30.
If your PI is not above 5500, you will get the monster 14,000 PI teams.
From 19 on, you can drop down, but keep your PI above 4000.
All these numbers are without any buffs active.
If you turn on any buffs, the PI requirements go up.
This should work for the current version 5.0.
It is the best technique I found so far.
gaming.stackexchange.com/users/113859/ChaoticGabibo
this might help as well 1 to 9 you can use your 3stars not ranked up or level up 9 to 12 - use 2 (3star champions that maxed out and must be above 1500pi) and another 3 star champ that is level 1 13-14 use three 4 star champs that should be 3/30 (on this level you will encounter KANGS TEAM or THANOS TEAM) 15-22 use three 4 star champs 3/30 (on this level you will encounter another players still controlled by AI
so be carefull with selecting your champs
make sure they dont trigger will power) 22 to infinity use a team of 4star champs that is 3/30 or maxed out 3starteam should be more than 4500 PI ( by the way on streak 20 to infinity you will encounter enemies that labeled easy but dodges, parry and block like a beast)
gaming.stackexchange.com/users/151424/ritter
Here is the method I use.
fights 1-8 weakest characters or highest 4 stars 9-12 lowest or maxed 3 stars 13-18 4 stars at 3/30 or better 19-infinity 2 4stars + 1 maxed 3 star or 3 3 stars maxed or 2 maxed 3 stars and a 2/20 3star this has gotten me to 100 wins.
gaming.stackexchange.com/users/169930/Eric Pease
I figured it out!
I have successfully done this repeatedly and failed when deviating.
Note that number of battle below is 1 less than win count on screen.
3v3 1-3 stars: 1-8
whatever 9... 3 star max or 3.5k total PI 3v3 3-4 stars: 1-13 whatever.
sand bag if you must (10-12).
14-15 4 star rank 3 or higher.
16... 3 stars max or higher no less than 5k total PI 3v3 1-5 stars: 1-15 whatever.
sand bag if you must (10-16).
16-18 4 stars rank 4 or higher 18... 4 stars rank 3 or higher (mix 2x4 stars rank 1x2) 3v3 4-5 stars:
Sorry I only have 4 star rank 4 and lower.
The requirements are too high for me.
(observation)
It seems to me, when the easiest option is double your total PI is when the infinite starts, but it wont kick in unless you have high enough PI.
gaming.stackexchange.com/users/188484/nowherevill
I really like Marvel Contest of the Champions, but all the information about this game is on forums. I find that the developers from  Kabam  want to keep some information a secret. And the information on  GameFAQs  is almost always outdated.  So, how do I keep a streak on 3v3 arenas?   Edit Question refers to 4* arenas.
How do I keep a streak on 3v3 arenas on Marvel Contest of the Champions?
gaming.stackexchange.com
<marvel-contest-champions>
gaming.stackexchange.com/questions/242093
gaming.stackexchange.com/users/113859/ChaoticGabibo
 1-8 weakest characters or highest 4 stars, 1 to 9 you can use your 3stars not ranked up or level up  9 to 12, 13-14 use three 4 star champs that should be 3/30. To achieve a streak on 3v3 arenas, in Marvel Contest of the Champions, the player has to figgure out a tactic.
Users explain their tactics to achieve a streak.