@Vagelisk 

I assume you are talking about Ipsec Site to Site??
yes you can do this by adding lan from Site C in VPN to Site A and B I won't even try to explain this in detail since it is quite difficult to explain.. I think there is a post about this on the forum.
but wouldn't it be easier to create a VPN between Site B and Site C?

@Clive_A might have some documentation on this

  @MR.S Thank you for your response. The problem is that at side A and side B i have omada controller but at side C i dont have. I will read the article that you sent me.

  @Vagelisk 

you don't need omada controller to create site to site vpn,
but take a look at the article,

  @MR.S I tried this between side A and side B but the problem is that side C has not omada controller or vpn omada router. It has a router named Oxygen. It has not a lot of settings inside to change. I have vpn connection between side A and side B (both of them has Omada controller and omada vpn router model er605) and i have also vpn connnection between side A and side C (as i said side A has omada controller and vpn router omada er605). Side C has the router that i said before. It can has only one vpn connection IPSEC and it has with side A. I was wondering if i can at least make a connection between two devices. I want to connect a device from side B with another device to side C. I tried with one NAT but with no luck.

  @Vagelisk 

I don't know Oxygen routers, but if it has IPsec then there shouldn't be any problems. As I said, IPsec can be run with and without Omada controllers and against many different brands. You got a link on how to do it, this description should work even if you have a different router brand.

I don't know if this is the right router but it seems like there shouldn't be any problem connecting it to an Omada router.

  @MR.S I tried it but doesnt work, thats why i thought if i can set up something inside omada controller to connect only these two devices one from side B and the other from side C.

  @Vagelisk 

No, it won't work.

If you can't create a VPN for your Oxygen router, replace it with an Omada router. It's not worth the time to try to do it.

  @MR.S Yes is exactly this router. this is in side C. When i made these settings from the article i lost the vpn connection. Maybe i have to add something at the table VPN tunel (Local subnet - Remote subnet)