Hellobee Boards

Is it for a specific event?

I think you have two options, an in ear model and one that covers the ears. I believe the in ear ones are more effective, you can ask if an audiologist will make a pair.

We used the over the ear ones when we went to Monster Jam, which is loud (even with the headphones and open air arena).

My clinic makes custom plugs that have noise filters in them- they're designed for musicians to block out the bulk of the noise but still allow them to hear what they're playing, etc. You should be able to find an audiology practice/hearing aid dispenser that makes them. They run about $120/pair though, so that might be a little pricy.
The other alternative is the over-ear phones, but they're noise cancelling. They're great to reduce noise/potential noise damage, but they *might* block out too much depending on the type of event!

We also have the Baby Banz. I think the larger size should be good for your 3 yo. I can fit them over my head and my kids were fine with them starting around 1-2 yo. We use them for concerts, sporting events, and noisy kitchen appliances. My kids were both sensitive to loud noises but have gotten less so as they get a little older. When I speak to my kids while they're wearing them, I just have to speak a little louder but they can still hear and understand, you can try them on and see how much noise they block.