Hello Zdenek
In order to get all traffic from the Internet to internal devices to be balanced, R2 and R3 must advertise both routes as equally desirable to the Internet at large. This way both options will be viewed as equal cost paths allowing for the routing to be divided more or less equally across both routers.
A similar configuration can be found at this Cisco documentation. This example shows how BGP will be used to load balance between two customer routers to two ISP routers. But you can switch this around and look at the configs of the two customer routers from the point of view of the ISP.
I hope this has been helpful!
Laz