combine with Its HP / speed make Brummbar has too much survivability and firepower.
The Brummbär is meant to be survivable because of the lack of turret. If you have a skilled opponent, it is very hard to kill it, unless you manage to set up a trap. Less skilled ones will often run it into too many AT units.
Overall I think it is in an okay spot right now. Previously it had 240 armor and was deemed too weak in survivability. I think balance team is going for a middle ground here. The only thing I find obnoxious is that it is really hard to snare, simply because running at it is damn dangerous.
Bouncing three shots in a row is quite rare as Vipper said. I am not sure though if this is the correct metric. It's quite fair to assume that you have at least 2 AT units on the field that can fire twice on the approach. I think the better question is 'how likely is it for these 4 shots to pen at least twice'. At this point the Brummbar usually retreats.
EDIT:
So chances to pen X times out of four shots in live/preview is:
0-1 (accumulated): 6,2%/4,5%
2: 23,2%/20%
3-4 (acc): 70,5%/75,5%
Now this attack will happen multiple times in a game (let's say 4 for the sake of it).
Beating all of them back has a chance of 77%/83% if your opponent retreats after two pens and 25%/32% if he retreats after three pens.
Additionally, all this neglects misses, so to make it short: I think you have a decent chance of encountering this somewhat regularly in the game. But on the other hand, the Brummbar cannot overextend too far because of a lacking turret.
I'd say let's see what the patch brings.