The first question to ask yourself is "Have I been keeping up on the watering?" Unless there has been very heavy rain on a regular basis since you planted it, it would need a thorough soaking about once a week, maybe twice a week in warmer weather. New plants always need regular watering to do well.

Ironically, the second question is "Is there enough soil drainage where I planted it?" In addition to water, roots need oxygen to grow, so after the soil is saturated in a watering, the excess water needs to drain away so air can enter the soil to provide it. Some pernicious fungi also like saturated doil, and Phormiums are susceptible to many of them.

The next question is "Is the soil rich enough to feed the plant?" Phormiums don't need huge amounts of nutrients, but poor sandy or chalky soil can lead to a lack of nitrogen, which can cause the older leaves to yellow, as they sacrifice themselves to support new growth, both of roots and leaves.

If it's just a few yellow leaves, those may just be old leaves whose time has come. Just clip them off, and don't worry.

You might want to inspect the base of the leaves also. On rare occasions, it will come home from the garden center infested with mealybugs, which can then build up to where they are weakening the plant. Look for little blobs of sticky "wool" attached to the bases of the leaves, especially the newest ones.

I hope all this helps!

Thanks for that I will cut 'em off and hope for the best!

Just the yellow ones, Total, and welcome to GoY!