I have mistakenly sent my LTC to (M) address instead of (3) address
Litecoin was having issues with users confusing Bitcoin and Litecoin addresses. To that end, they introduced a new address format for Pay-to-Script-Hash (P2SH) addresses. This new address format encodes the address with an M prefix rather than the original 3 prefix which is also used by Bitcoin's P2SH address format.

As any underlying address can be expressed in either address format, each address with an M prefix has an equivalent address with a 3 prefix. They simply are two different encodings of the same P2SH address.

Check out the address in question on the CryptoId block explorer for example:



When you search the address in question on this block explorer, you should be able to see both the M address that you meant to send to as well as the 3 address that your address now changed to, just like for the random example in the image.
 A transaction output will arrive in the same wallet with either of these two addresses given as destination.