First off, you typically want to ignore those preapproved notices. In fact, you may want to look into opting out of those when possible because they are doing a soft check on your credit that can actually hurt your credit score slightly for a few months.
However, what is causing your score to be low is definitely the loan. It's a newish account still and adds a large balance. The age is the fact that it has only a short record of payment so far. The number is in the fact that you have three accounts and don't have any records of not paying something in full yet. Keep things as they are, once that loan is paid off, your score is likely to end up higher than the original position since the successful payment and closing of the loan will boost it.
Also, bear in mind that 650 isn't something to worry about yet. You're still establishing your credit, but what you do have is good. The rest just takes time.
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However, what is causing your score to be low is definitely the loan. It's a newish account still and adds a large balance. The age is the fact that it has only a short record of payment so far. The number is in the fact that you have three accounts and don't have any records of not paying something in full yet. Keep things as they are, once that loan is paid off, your score is likely to end up higher than the original position since the successful payment and closing of the loan will boost it.
Also, bear in mind that 650 isn't something to worry about yet. You're still establishing your credit, but what you do have is good. The rest just takes time.