Hi volpone, I’m certainly not the lightning bolt expert but I wish I was so I could help you 
I only read the comments by Pi Engineers that I linked earlier where they mention it’s an under voltage indicator and I followed their advice and removed the unnecessary thin wiring which they say may cause the voltage drop, between the iPower 5V and the Pi3 - and that fixed it.
Are you using a 5Vdc splitter cable or using the ribbon cable that comes with the 7" screen?
Do you have the official Pi charger? It’s 5.1V, where that extra 0.1V is to account for potential voltage drops.
Is there anything else between the iPower’s output connector and your Pi3’s microUSB port? Extender cable? In line switch?