Hey there. The challenges release daily, we are still technically on Day 2 so only two challenges have been made available so far. I don't think there was ever a bug, just people not knowing this. The only other thing could be if a player's power is below 1,750, challenges may not appear.
For you though, wait until today's reset for challenge 3, and tomorrow's reset for challenge 4.
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Maybe it's where I normally play at the end of the week and all challenges are there. Thanks for the heads-up thought it may have been an issue.