Hey - I can help someone! I've been where you are for about a week and finally got the hang of it. You have to be exact on your barrages - exact center of the ERs.
The attack order is below:
Barrage on 1st ER, Barrage on 2nd ER, 1 Arty each on all three ERs. This should pop the 1st two ERs, giving you 15 more GBE. Use that to pop the 3rd ER with a third barrage. No troops needed.
A caveat is, if someone puts an ER right by the beach, your troops will be faster to that ER than barrage to the back of the base so that is a consideration.
Now, the strategy for chests: You should purposefully bounce between max level 25 and zero stars on Leg1. Usually you will start at 0 stars on Legendary 1. You will lose that battle against an ER4 or 5, so just BART down one ER and quit. You will then have 1 star and be at Max Level 25. The next base will be ER3 doing the same tactic as you. If you win, you go back to Leg1 with 4 stars. Repeat the strategy. If you lose, you will have to win 2 battles to get back to Leg1 (3 stars each). Repeat this strategy until you get your chest, but DO NOT CLAIM THE CHEST until you are in Legendary 1 to receive max funds.