The word trefoil comes from the French "trois feuilles" or "three leaves". It sort of resembles clover.
You can make it interweave by following steps 4 and 5 for the simple triquetra, next.
I find it just as easy to draw the trefoil all in one swoop without the dots and lines. Some people find it easier to begin with a curved-edge triangle. See what works for you.
Step 4 is nice to use as a string just as is, with or without a square border.
You can see a two-pencil string trefoil here. It's the last picture.
These directions are drawn in pen so you can see them better, but REMEMBER: steps 1-4 are done in pencil. These lines act as your string, although you don't draw on them or up to them. Read on, you'll see.
Don't be wimpy with the arcs. They need to really swoop towards the third dot. This will give you a nice little triangle in the center.
Step 5 is when you start using ink. The short ink lines are to indicate to yourself what goes over and what goes under. I've drawn them bolder. The lines you draw later to connect everything are in dotted red.
Step 6 shows the triquetra with the pencil string in the middle of the lines. Now you can start embellishing!
You can see my simple triquetra posts here and here.
SINGLE LOOP TRIQUETRA
Again, these directions are in pen so you can see them better, but REMEMBER: steps 1-5 are done in pencil. These lines act as your string.
Step 4 shows the looped line in red, just so it's clear.
Step 5 shows the entire pencil string.
Step 6 is when you start using ink. The short ink lines indicate what goes over and under. I've drawn them bolder. The connecting lines that you draw later are dotted.
You can see a single loop triquetra in my post here, as well as two more simple ones.
(I think step 2 looks like an unhappy cyclops.)
Here's a nice picture of a looped triquetra.
This one takes quite some concentration, but it's essentially the same curvy line drawn three times. The first picture below is the basic line. It's actually rather simple; it's when they all start overlapping that it becomes confusing! In the next picture you can see the three lines, meeting at the corner dots. I've done them in solid black, dotted red, and dotted dark blue so you can see how they intertwine.
This one is easier than the looped triquetra, and it's the most recent one I've tried. I think it's my favorite. Here's a picture of a lovely, carved wood, cloverleaf triquetra I found on Etsy here. You can see my cloverleaf triquetra zentangle tiles here.
As with the looped version, this, too, is the same line drawn three times.
Don't be discouraged. Smile. Breathe. Relax. Have fun!