Should I split a 3MByte block into smaller blocks if using XXTEA?
You should not use XXTEA, especially in its large-block variant. It's reported as broken, and that seems very credible. Some of the problem is with the overly aggressive $6+52/n$ rounds, but beside this, the cipher has not attracted wide review.
This question discuss other variants of TEA.
Except in the specific cases that you need an all-or-nothing bijection of a block (because you can't afford that encryption makes the ciphertext any larger), or code space is extremely scarce, my recommendation is to use a standard authenticated encryption cryptographic primitive such as AES-GCM. Libraries for that are available in most major languages.