三个交换机分别为S1、S2、S3，两台PC分别为PCA和PCB，如果两台主机都属于不同的VLAN，要想实现主机之间的互通，三台交换机上需要怎么配置，用H3C

要实现不同VLAN下的主机之间的互通，需要进行以下配置：

1. 在交换机上创建VLAN并指定VLAN ID，例如创建VLAN 10和VLAN 20：

[Switch] vlan 10
[Switch-vlan10] vlan-id 10
[Switch] vlan 20
[Switch-vlan20] vlan-id 20

2. 配置交换机的端口为Trunk口，使得可以传输多个VLAN的数据，例如将S1的1号口和S2的1号口配置为Trunk口：

[Switch] interface GigabitEthernet1/0/1
[Switch-GigabitEthernet1/0/1] port link-type trunk
[Switch-GigabitEthernet1/0/1] port trunk allow-pass vlan 10 20
[Switch] interface GigabitEthernet1/0/1
[Switch-GigabitEthernet1/0/1] port link-type trunk
[Switch-GigabitEthernet1/0/1] port trunk allow-pass vlan 10 20

3. 配置交换机的端口为Access口，使得主机可以加入指定的VLAN，例如将PCA连接的端口配置为Access口并加入VLAN 10，将PCB连接的端口配置为Access口并加入VLAN 20：

[Switch] interface GigabitEthernet1/0/2
[Switch-GigabitEthernet1/0/2] port link-type access
[Switch-GigabitEthernet1/0/2] port default vlan 10
[Switch] interface GigabitEthernet2/0/2
[Switch-GigabitEthernet2/0/2] port link-type access
[Switch-GigabitEthernet2/0/2] port default vlan 20

4. 最后需要在交换机上配置路由，使得不同VLAN下的主机可以相互通信，例如在S1上配置路由：

[Switch] ip routing
[Switch] interface Vlanif10
[Switch-Vlanif10] ip address 10.0.0.1 24
[Switch-Vlanif10] quit
[Switch] interface Vlanif20
[Switch-Vlanif20] ip address 20.0.0.1 24
[Switch-Vlanif20] quit
[Switch] interface GigabitEthernet1/0/2
[Switch-GigabitEthernet1/0/2] port link-type access
[Switch-GigabitEthernet1/0/2] port default vlan 10
[Switch-GigabitEthernet1/0/2] arp broadcast enable
[Switch-GigabitEthernet1/0/2] quit
[Switch] interface GigabitEthernet1/0/1
[Switch-GigabitEthernet1/0/1] port link-type trunk
[Switch-GigabitEthernet1/0/1] port trunk allow-pass vlan 10 20
[Switch-GigabitEthernet1/0/1] quit
[Switch] interface GigabitEthernet2/0/1
[Switch-GigabitEthernet2/0/1] port link-type trunk
[Switch-GigabitEthernet2/0/1] port trunk allow-pass vlan 10 20
[Switch-GigabitEthernet2/0/1] quit
[Switch] interface Vlanif10
[Switch-Vlanif10] ip address 10.0.0.1 24
[Switch-Vlanif10] quit
[Switch] interface Vlanif20
[Switch-Vlanif20] ip address 20.0.0.1 24
[Switch-Vlanif20] quit
[Switch] ip route-static 20.0.0.0 255.255.255.0 192.168.1.2
[Switch] ip route-static 10.0.0.0 255.255.255.0 192.168.1.1

其中，192.168.1.1和192.168.1.2分别为S1和S2连接的路由器的IP地址。此时，PCA的IP地址应该为10.0.0.x，PCB的IP地址应该为20.0.0.x，可以通过ping命令测试它们之间的互通性。