Also entirely depends on your situation and the context the question is being asked in. If you only have 2 trade lines period on your credit report, and you've only had credit for 12 months or so, then chances are you might need the 3rd one, or just more credit history period, in order to get an approval of some sort (automated or manual). But if you have 30 years of credit, 800 scores, but only 2 open credit cards and nothing else... then there is really no need to get more credit or open another trade line.
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