Is it possible to connect several networks into one? I have read articles about site to site configuration through IPsec. But I haven't seen any articles which explain if it possible to connect 3 or more networks into one.
Networks which are connected are called an internet. The largest collection of connected networks is the public Internet, with millions of connected networks.
You can tunnel across intermediate connected networks using various protocols, including IPsec. There is no reason to think that you cannot have multiple tunnels from one network to other networks. Depending on the technology and protocol(s) used, you can even set up a cloud to which many sites connect, e.g. an MPLS VPN.