When you navigate from the first tab (based on table/block A) to the second tab (based on table/block B), Forms perform validation and - as not all required fields that belong to table A (the ones that are placed to the third tab) aren't populated - Forms raises an error.
One option would be to remove the "Required" property from all fields that belong to table A and are placed to the third tab. It means that you'd have to check whether they are populated manually, i.e. write a trigger which would do that. Which one? PRE-INSERT and PRE-UPDATE might be your choices. Code would look like
if :tab3.field1 is null or
:tab3.field2 is null or ...
then
message('Not all fields have been populated on tab 3');
raise form_trigger_failure;
end if;
Or, rearrange the layout by
- moving fields from tab 3 to tab 1, or
- switching tab 2 and tab 3 positions
Or, if the process goes like that, maybe your data model is wrongly set. What you described looks like some kind of a "master-detail" relationship, where table A acts as "master" and table B as "detail". You shouldn't populate master partially, especially not if all its columns are required. If that's the problem, well, then it really is a problem as changing data model doesn't necessarily be simple as it affects quite a lot of things.