"is one third of" is correct and usable in written English.
You could use it to compare two numbers and show that one is one third of the other. For example, "6 is one third of 18."
Its size is one third of the CN HCCB-TBM.
The achieved beam power for T2K is 240 kW which is one third of design value of 750 kW.
The long-term performance analysis shows that the increase in soil temperature is one third of that without heat recovery.
The average crystallite size of the PVA protected nMo2CN nanospheres (6.1 nm) is one third of that of the PVA-Mo2C/Mo2N without PVA protection (17.9 nm).
This means that the area with useful relays is one third of the area of a conventional sector.
For fractional frequency reuse scheme the feedback load is, that is, one third of the load for the proposed algorithm.
Still, $15 is one-third of the typical phone bill.
Union officials say production is one-third of what it was before the walkout.
Lewis Durham, 20, is one-third of sibling trio Kitty, Daisy & Lewis.
The cost is one-third of what a sewage system would cost to operate.
Today, the post-George W. Bush, post-Mitt Romney conservative movement is one-third of the way there.
