Kailyn Lowry says "nothing compares" to her sons.

The 25-year-old "Teen Mom 2" star posted a picture of 7-year-old Isaac, 3-year-old Lincoln and 3-week-old "Baby Lo" on Isaac's first day of school Tuesday.

"Nothing compares! #mythreekings," she captioned the snapshot on Instagram.

The photo shows Isaac putting an arm around Lincoln as their baby brother sleeps in his car seat. Lowry said in a later post that Isaac was "so nervous" to start second grade, but added he ended up having a "great" day.

"I can't believe my boys are growing up so fast!" she wrote. "My 2nd grader went back to school today, and it was so emotional saying goodbye! He was so nervous this morning, but he had a great first day and he's so excited for the rest of the year. I'm so proud of him!"

Lowry welcomed Baby Lo on Aug. 5, but has yet to officially name the infant.

She shares Isaac with ex-boyfriend Jo Rivera, Lincoln with ex-husband Javi Marroquin, and Baby Lo with ex-boyfriend Chris Lopez.

"[I'm] positive he'll be around," the star said of Lopez in an interview with Radar Online published this week. "I think every child deserves a dad."

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"He does the best to his ability even though it doesn't necessarily meet the standards of what everyone else expects of him," she added of Lopez's role in their son's life. "I think people need to keep that in mind."

Lowry came to fame on the MTV series "16 & Pregnant" before starring on "Teen Mom 2." The show co-stars Jenelle Evans, Chelsea Houska, Leah Messer and Briana DeJesus.