Storing public certificates and private keys in AWS

Question

I have a use case where I want to usecertificate based authentication in AWS Lambda to generate oauth tokens. Currently I am storing the certificates and private keys locally and running it like a normal java application.

However while migrating to AWS Lambda, I need to store these .crt and .key files somewhere in AWS, so that I can use it in AWS Lambda. I have come across some solutions like

1. Storing certs and keys in S3 buckets

2. Storing certs and keys in Secrets Manager

3. Storing them in Amazon Certificate Manager**

Can someone please help with which method is efficient and optimal for storing certs and keys?