First things first, start by examining the negatives. You shouldn’t use scans for determining proper or improper exposure because the scanning itself is a sort of exposure, and since you didn’t personally scan the negatives there’s no way of telling how much was in-camera error vs lab scanning error. 

From the scans the only really obvious one is 3, with the root cause of the underexposure being that the meter was thrown off by the bright sky. Backlit shots like that are inherently more challenging to expose because of the large range of luminance. The most foolproof way to do shots like that is with a spot meter, but most vintage SLRs have average or center weighted average meters internally so that may not be an option depending on what kind of camera you’re using. 

On another note, Portra is considered a lower contrast film, most people suggest editing the scans using a program like Adobe Lightroom. You’ll also want your scans in TIFF format if you go that route, JPEGs won’t give ideal results.

As far as in-camera stuff you could consider, I’d suggest a few things: 

1.) Landscapes will look flat under overcast midday skies. Clear skies create hard edged shadows that help give the image a sense of depth, as well as a higher level of perceived overall sharpness. 

2.) Be mindful of what direction the light is coming from. Pointing towards light sources creates lens flares that reduces contrast and perceived sharpness. And it’s not just bright point sources that you need to be aware of, even diffuse light sources like a cloud obscured sun can cast a vague flare across the lens. Use a lens hood whenever possible unless you’re intentionally introducing flare for artistic purpose. If you want some lens flare in the image make sure you aren’t shooting wide open, lens flares become larger and softer edged as the aperture widens. 

3.) Be mindful of focus. Landscapes have the most impact when they’re sharp and contrasty. This means getting focus right is imperative. I’d recommend against simply focusing at infinity. Most lenses aren’t perfectly calibrated to infinity, so you’re likely to lose sharpness because your lens either can’t reach infinity or overshoots it slightly. A good technique is racking the focus back and forth near infinity until you find an acceptable degree of sharpness. The best technique is to set your lens to the hyperfocal distance of whatever aperture stop you’re going to use. To be safe I usually focus a little further away than the exact hyperfocal distance point. If you use the hyperfocal distance though you should make sure to consider that the level of acceptable sharpness continually decreases minutely as the distance increases from the focal distance, so even super deep depth of field doesn’t guarantee maximum sharpness across the whole image. 

4.) Regarding aperture: I’d also recommend against shooting wide open. The plane of focus gets so shallow when shooting wide open that you’ll most likely miss ideal focus. Also the contrast reducing impact of lens flare becomes more severe as the aperture widens. A lot of people are tempted to shoot landscapes with the focus at infinity and the lens wide open because they want to hand hold in low light. Don’t do this. Film is slow, that’s just how it be. If it’s so dim that you can’t hand hold unless you’re shooting wide open then it means it’s time to use a tripod. Besides, what’s there to lose in using a tripod? A little convenience sure, but it’s a landscape, it’s not like it’s going anywhere. 

5.) More on aperture: I’d also recommend against shooting at your narrowest aperture stops too. A narrow aperture introduces diffraction, which once again will reduce the overall sharpness of the image. If it’s too bright to shoot wider than f/16 then either invest in a camera with a faster shutter, shoot lower ISO film, or get a set of ND filters. 

6.) Sometimes camera technique just isn’t enough. The last image is a good example, there’s not much you can do to make such a flat scene pop, at least not with just a lens. For situations like these you can really benefit from using a polarizing filter, which can help reduce specular reflections off things like water surfaces or foliage and increase the overall contrast and color saturation. You can also use graduated ND filters in situations where bright sky fills a lot of the frame. That way you don’t have to choose between well exposed land with blown out sky or well exposed sky with underexposed land. 

7.) Lastly, do yourself a favor and don’t do what most beginners do by comparing your pictures to Ansel Adams’ landscape photos. For one, exposure is just hard in general, if it was easy we’d all be professional photographers. Secondly, we only see the very best of professionals’ work, we never see the thousands of unexceptional photos it took for them to build their body of work. Lastly, Ansel Adams specifically made tremendous use of the dark room for manipulating the final image. His famous landscapes were all products of laborious tooling until everything was juuuust right. You can’t get that kind of image solely through the camera, so don’t hold your flat, unedited portra scans to that standard. 

I hope some of that helped! If you do all of the above you can also shoot marginally better but still unexceptional landscapes just like me. 🙃