Should I use HMAC or KMAC for SHA3?
So if I want to implement a MAC function Can i just do the first method ?? just pad the message with key and do the hash ?


Yes, $mac = \text{SHA3}(k||m)$ is a secure MAC if $k$ is a fixed-length key. This is an explicit design goal of SHA3.

You can also rely on HMAC or KMAC instantiated with SHA3. These will also give you secure MACs but the added complexity is not necessary.