Mitigating private key loss for the average non-technical user
Splitting your key by just splitting into partial strings (doesn't matter if it's every 3. symbol or consecutive symbols) is a terrible idea, because you hand out actual information about your key - to them and to everyone who compromises their system.

For RSA this is actually a real problem, because partial knowledge of the private key makes Coppersmith's attack possible. In the original attack the known bits had to be either the least or most significant bits, but the attack has been refined quite a bit. For example, Reconstructing RSA Private Keys from Random Key Bits by Heninger and Schacham (2009) just need a fraction of $0.27$ random bits of the private key and a small exponent. In that paper the related work section gives a good overview over previous works.

As a refinement for the splitting:
I suggest looking into secret sharing schemes (e.g. Shamir's secret sharing), so that at least no one with less than 3 shares can't recover any information about the key. But still, the security of your key becomes dependent of the other parties keeping your key secure and not be malicious. And of course you need a stragety to change your key and update their shares. Reasons for this might be that you want to change the list of trusted persons, one of them lost all the date on their computer, etc.

In general, the dependency on their security is a practical issue: If you use proper implementaions of current state-of-the-art encryption schemes, it is much more likely that key management and storage contains a security weakness than the actual encryption.

Regarding the last part: If you worry about one friend loosing his data at the same time as you (roughly the same time, before you could redo the splitting process), secret sharing schemes can be set up so that you need $k$ out of $n$ shares to recover the key. And it doesn't matter which $k$ of those $n$ shares you use.