VirtualBox set Guest interface as a bridged adapter

A bridged adapter means that the Guest adapter is bridged to the Host adapter i.e. it can see the whole network (request address from DCHP server etc)

VBoxManage modifyvm my-guest-name --nat1 bridged
VBoxManage modifyvm my-guest-name --bridgeadapter1 eth3

nat1 is the first interface of the Guest machine.