<p>I'm trying to model the threads of a &quot;Poland Spring&quot; 500 ml bottle so I can 3D print an adapter for it. But I can't find information about it. I emailed them but they said they didn't have the information.</p>
<p>How can I find this information out?</p>
<p>The bottle seems to use non standard threads. It uses 3 threads 120 degrees apart that does not go all the way around.   Any information on how to get this information?</p>
<p><a href="https://i.stack.imgur.com/nEnwV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nEnwV.png" alt="bottle" /></a></p>

You can use a program known as
OpenSCAD
with the
threads library
to assist your objective.
The important aspects of a thread are the major diameter (outside diameter at the thread surface), the length of the bolt/nut and for your project, the start count. The noted library has a parameter called n-starts, which covers your triple start requirement.
It may require some trial and error processing, but you have a good foundation with OpenSCAD library.
Even if the bottle is manufactured with Imperial reference (unlikely), you'd probably have an easier time with metric threads. I just checked a Dasani water bottle, which measured at 27.27 mm major diameter, 10 mm length, thread pitch 2 mm. The thread pitch in metric is the distance between peaks (or valleys) from one to the next. This particular bottle has an interrupted thread, but that should not be a factor for creating an adapter.
This Dasani bottle does not have a multi-start thread. It's not a complete thread, but it is single start. Considering that a water bottle top has minimal length, you may also have an interrupted rather than a multi-start thread.
The code for the measured thread in OpenSCAD is relatively simple.
use <threads.scad>;
thread_dia = 27.27;
thread_pitch = 2;
thread_length = 10;
metric_thread(thread_dia, thread_pitch, thread_length, n_starts = 3);
The above code would generate a 3-start thread in the form of a bolt. Using a difference() code would provide for the inverse required for your proposed adapter.
top image is single start, bottom is triple start.
Top view comparison: