I’ve been using Jerry’s Soho Urban Artist’s Oils for the last year or so, they’re an affordable, decent student-grade oil paint and a number are single pigments. Though I use a hybrid of Carder’s Geneva and Zorn palletes, they do offer the true primaries;
Primary Cyan (PB15:3, PW6, PW4) Phthalo Blue GS and White
Primary Magenta (PR122) Quinacridone Magenta
Primary Yellow (PY3, PW6) Hansa Yellow and White
And they’re earths are hues of PR101 and PY42.

They offer a complete starter set for a hundred bucks; paints, medium, brushes and table easel if you’re stuck away from your usual supplies and stuff.

In the current unpleasantness I don’t know how their shipping is right now, …but the website is up.

The most simple cmyk palette I know is composed of Prussian blue, cad yellow medium, Alizarin (or a good substitute), and white. These colors are recommended in Jose Parramon’s book, The Big Book Of Oil Color. In the color section of his book, he presents a couple paintings and a couple of pages of recipes for various colors. This palette is an excellent opportunity to learn to mix color.

what would you suggest for 3 opaque primaries?

For blue you could use a heavily pigmented Phthalo Blue, as it will be almost black in masstone, but will keep it’s chroma very well when mixed with white.

I like to use these as primaries:

Prussian Blue PB 27
Quinacridone Rose PV 19
Indian Yellow PY 110

Each color here has a specific reason for the choice and inclusion. First, the “blue” is more like a cyan in usage but has a very real blue look with just a tiny touch of white, but with lots it will go all the way through to slightly muted cyans. This allows a lot of versatility in usage.

The Rose functions as a vibrant and intense Magenta but leans slightly more toward a red, this allows it to mix good violets mixed with the “blue” but also allows it to mix excellent oranges with the yellow.

The yellow specifically leans orange, but the tiniest touch of the “blue” will also allow it to make more neutral yellows (though less vivid than say a Cadmium Yellow). The key is, the most important ranges of colors are very mixable with it, greens with more of the cyan/blue or oranges with the Rose/red. This gives it a wider range, where a cadmium yellow mixed with most magentas will give much less vibrant orange mixes than this.

Transparent pigments provide a much wider range of possible hue intensities where opaque pigments (like cadmium yellows) will reduce the mixing range. This means that if one is using opaques, they should have a wider range of pigments available so they have greater mixing range to work with.

Good luck with your limited palette of colors. If you can afford a 4th color, adding in an orange leaning red like PO 36 or PO 73 will allow you to switch to a green leaning yellow like Cadmium without loss of the orange/red range of color mixes quite well.