Trouble is probably caused by the Synthetic key (Syn1), because you probably don't get data association you need by combining all these fields.
When creating table associations, it's always the best to associate by just a singe field (which can also be a composite key = combination of multiple fields).
It really depends on what you want to achieve, but I would first suggest you use Concatenate statement to put all 3 tables into one and see how that will work for you.
Second option is to use Qualify statement, to differentiate fields with the same name in different tables and only use the "Relatienr" to associate them.
Hope this helps,