How can I connect three different branches over secured VPN connection using static IP's to be in the same LAN?

By Configuring EZVPN on the Three routers which is the most simplest way to do such a connection.

Most secure way is to have a Leased line connection from your ISP

Second : to have one of microsoft servers like2008 in both of your branches and configure a VPN connection between your branches.

I doubt that you will be able to do that.

How you will route between branches and they are in the same network?

Traffic will not reach the gateway, since they are designated to the same network.

The workaround solution for this is to subnet your network, they configure your branch router to statically route the traffic to the other branches.

IPSec based or SSL based VPN solution will accomplish this. If you subnet the network.

 Of course you can do that and by the way this is the method that is used in Banks for centralized internet connections ,so all branches takes internet from the head quarter and this will keep internet  being monitored and the servers are communicating to each other inside this network as one lan.

You will have a head quarter Firewall and you will have two branches firewall such as juniper  SRX240 For the ,headqueater and juniper  SRX100 for the branches what you will do is :

 A-Create an IP SEC VPN on the SRX (240 ) considering it as connection point (multipoint)

B-Configuring IP SEC VPN on the two Branches SRX as Site to Site VPN and identifing the Lan networks I have done it many times and it will in a perfect way ,it's really amazing 

Juniper have a greate example , the configuration commands looks hard but doing it from the web is much easier 

http://kb.juniper.net/kb/documents/public/junos/jsrx/JSeries_SRXSeries_Multipoint_VPN_with_NHTB_12.pdfI hope this will answer you question 

You can check step by step details on Official