So basically using multi-Sig cold wallet on airgapped machine provides the ultimate security?
With using a multi-signature wallet, you may increase your security, but I recommend you use it if it's really required.
Multi-signature addresses are usually used when a transaction should be signed by multiple parties.
If you generate a single-signature wallet on an air-gapped device and keep your keys safe, it's secure enough.
I was thinking what if someone generate the same private key as my address in case of single address (which is very very unlikely) but using multi-sig makes this impossible,yes?
Whether you use a single-signature address or a multi-signature address, that's impossible.
If an address is multi sig of say 3 address then attacker has to find 3 private keys correct?
It depends.
If the address on is 3 of 3 multi-signature address, all the three private keys would be required.
If the address is 1 of 3 multi-signature address, a single private key would be enough.
Also bitcoin send to individual address which generate multi-Sig can also be spent individually right?
Yes. Each of private keys used for generating the multi-signature address can be used to generate a single-signature address.