If you really need a TAP-style connection, you would need to specify the
server-bridge option without further parameters - this would enable bridging mode and OpenVPN pass on DHCP requests (DHCP proxy mode). You also would need to bridge your tun interface with whatever your LAN interface is using
brctl addbr br0
brctl addif br0 eth0
brctl addif br0 tun0
ifconfig br0 inet 192.168.3.99 netmask 255.255.255.0
see the server-bridge option description in the OpenVPN documentation for details.
But honestly, you should rather route than bridge whenever possible - it allows for better debugging and less unnecessary network noise (broadcasts) transmitted through your VPN.
For this case, a config file containing the
push "route 192.168.3.0 255.255.255.0" on the server and the accompanying client-style config file (specifying the
client option) would do. Example:
local <your public IP address>
port <your OpenVPN port>
# reduce MTU if necessary
# tun-mtu 1400
# place your cert/key/DH paths here
# OpenVPN client network
server 192.168.99.0 255.255.255.0
# push client route
push "route 192.168.3.0 255.255.255.0"
keepalive 10 60
# logging options
See the documentation for details on option parameters.