iPhone simlock

Matsm

Registered
Looking for a little help here , got an unlocked (supposingly ) iPhone last year and trying to use a three mobile sim and even the sim not valid message . Ok seems simple enough it's not locked however the phone works with an o2 sim and Vodafone sim so would suggest it's unlocked so I'm failing to grasp why three sims are not supported also how do I go about getting this fully unlocked ?
 
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