What's the best way to get multiple avenues of monitoring in one trigger?

[MaeveFirstborn]

My team and I have a Zabbix server configured for a client with 3 sites connected together in a VPN. One of the sites has really poor internet and so our gateway monitoring tools sometimes flag it as being down when it really isn't due to slow responses (as a consequence of a QOS setup that prioritizes other traffic). My boss had the idea that we could set things up so that for each firewall gateway IP, we ping it not from just that firewall, but from each of the other firewalls or zabbix proxies at the other two sites. Conceptually this isn't that hard, but what we'd like to do is make it to where when we get a trigger notification from one, it includes data from the other two sites.
Is there a way to have Operational Data in a trigger include information collected from another other hosts' items without being a dependency in the key expression itself?
One thing I'm looking at is using External Checks and creating a script that correlates this data somehow, but I wanted to see what the other options are.

[cyber]

In the event you can use data from items, that you are using in expression. So you need to construct your expression in a way, that other items do not influence the end result. Then you can refer to all used values as {ITEM.VALUE1}, {ITEM.VALUE2}, {ITEM.VALUE3},..., {ITEM.VALUE<N>}. Like... for one item use the 0/1 check, what matters, but for others something like bytelenght>0, then it really does not matter, what are those other values, they just need to exist...

[MaeveFirstborn]

That's helpful, thank you. Follow up question from my boss, what is the general philosophy for using Zabbix to monitor multiple sites?
What are the conventions for distributed monitoring and monitoring access of resources in one site from another? I.e. to make sure that you're not just monitoring availability from a host according to that host itself but also that other sources have good access?

[cyber]

Can tool have a philosophy? They say, monitor, what matters... If this is what you need, you need to build your monitoring accordingly..
But maybe there is someone from Zabbix, who can answer this..

[troffasky]

If your server is at Site A and you want to monitor connectivity Site B<->Site C then you need a Zabbix proxy at either B or C.
You can "get clever" by doing things, for example, a carefully-crafted port forward on Site B to something at Site C, which you can then monitor from Site A. But it will not give you the full monitoring that a Zabbix proxy would.