Why traceroute is not resolving an IP address

Hi.

I have 3 computers, say PC1, PC2 and PC3, with debian installed on each. Each of these 3 computers has 3 ethernets.

I have connected PC2 to PC1 using eth0, and also PC2 to PC3 using eth1. I have configured DHCP on PC2 to give an IP to PC1 and PC3. According to command ipconfig all seems to be ok.

I can make a ping from PC1 to PC2 and vice-versa. I can also make a ping from PC2 to PC3 and vice-versa.

However, when I try to ping PC1 from PC3 or if I try to ping PC3 from PC1 it says network is unreachable.

Also, if I ping the IP of eth2 of PC2 from PC1 it says network is unreachable. Looks like PC2 don't have the forwarding activated, however I executed sysctl -p /etc/sysctl.conf and it says:
net.ip_v4.ip_forward = 1

So why packets are not being transmitted from one network to the other?

Thanks.