Any good ideas ? Small PC boxes I could buy? and combine 3 network cards ?
It all depends on the Internet connection speed and what and how many packets you have installed on that box!
It could be that a smaller variant likes the SG-2440 would do the job also fine, and with more luck a smaller one
will do the job too but without installing any packets then on that device. (pure firewall, DHCP, 3 VLANs,….)
SG-1000 - up to ~100 MBit/s Internet speed and low VPN & firewall only
SG-2220 - up to ~500 MBit/s, Internet speed IPsec & OpenVPNVPN, IDS/IPS
SG-2440 - up to ~900 MBit/s Internet speed, VPN, Squid & SquidGuard, pfBlockerNG, IDS/IPS
Or a total different way with the R1 from netgate.com