Steps to reproduce:

1. Create a trapper text item and a trigger with a condition using this item:

{host:trapper_text_item.strlen(#1)}>0 and {host:trapper_text_item.strlen(#2)}>0 and {host:trapper_text_item.strlen(#3)}>0

Thus, we would like to fire this trigger when 3 last values of this item are not empty.

Based on https://www.zabbix.com/documentation/5.0/manual/appendix/triggers/functions,  we can expect it, because:

⇒ strlen()(is equal to strlen(#1)) → length of the latest value
⇒ strlen(#3) → length of the third most recent value

2. Send 3 empty string to this item (just to collect enough data):

3. Send 1 non-empty string to this item:

Result:
The trigger was fired:

Expected:
The trigger wasn't fired because the second and the third most recent values have empty string and their length is 0.