We aren't able to fully answer this question without being able to examine the weapon. That said, help us understand as best you can.
- How long have you been fighting Bel? Building weapons? What's your annual exposure to other veteran fighters and building/checking techniques?
- Did you hit on a sweet spot?
- Describe the etiology and diagnosis of the injury. Was the kid frail?
- Was it just a stroke of bad luck?
If the weapon is passing hit tests, I would say it needs to be checked again, and preferably by veteran fighters who know what they're looking for. Your weapon may indeed be perfectly fine, but we won't know unless someone in-the-know takes a peek.
I've seen reds with four layers of blue pass, but three seems like it could be too few - again, without seeing the weapon and being hit by it, it is an unknown.
If you're in doubt of its safety, pull the weapon.The problem with all blue foam on large weapons is that there is a diminishing return on blow cushioning as more layers are added. That's the beauty of construction techniques that utilize progressive foam densities.
Personally, having built reds, spears, and javs with progressive foam densities and advanced biscuit/stab tip techniques, I would never go back to just blue foam because I realize the quality and safety difference. Were I you, I would say that if I couldn't afford better materials then I wouldn't build large weapons. This is done entirely out of respect for the safety of my friends who are graciously allowing me to assault them with potentially deadly weapons.
That said, a roll of yoga mat can be had for $20 on sale in places like walmart and kmart. A package of puzzle mat floor tiles can be had for ~ $25 and you get 4, 2'x'2 sheets. Yes, you do pay a lot up front compared to a six or seven dollar roll of blue, but you can make a lot more high quality weapons if you pony up the cash. Perhaps consider it a realm purchase where everyone contributes and gets use of the materials.