Q.Meyer lemon tree planted outdoors
My tree was planted 3 years ago but has not flowered. It is 6′ tall with beautiful green leaves with no sign of flowers. Please help.

A healthy Lemon Tree should began producing in it's 3 year.
Your tree is just getting old enough to fruit.
Citrus need plenty of warm sunshine, consistent watering, fertilizer schedule and pollination to grow.
Here are some articles to help refresh you and do a check on the growing conditions.
https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm