DOF is good, and it's sharp, i'd probably crop this slightly too
Think you picked the best 3 from the bunch.
It's hard to judge the 2 as they are snapshot types of photos and with my own pets, you don't always have time to compose a shot but i'll crit them like I would any others.
1) Be aware of your surroundings, the pole in the bottom left is a little distracting. Good focus on the eyes, looks clean and crisp. I'd be tempted to crop the the bottom and left side out, it'd remove the pol and your jeans/feet.
2) Again, not a lot you can do with a dog in the background too, it's not always possible to have a none distracting background. Focus is again on clean and sharp.
3) I will lay into this one a little more because it's a set up static shot. Bucket shouldn't be there, some will complain about the top of the hedge as well. If you'd moved half a step left or right and not shot straight on we wouldn't see your reflection in the plastic. Also looks like it could do with a clean and a polish
You also learn everyone is different. Somethings every one with agree on, others will cause a discussion about it.
Good start overall, it takes a lot of practice to see the details in everything and a photo is as much the background as the foreground. A word of warning, if you like looking at photos don't get into photography seriously, all you'll do is pick faults with every photo
, yes it is hard to get everything detracting in the background out of the way when its a quick shot of your pets lol, but i'll try my best to keep them away from objects lol 
There are certainly principles and guidelines and a lot of technical stuff to get your teeth into, but use it as knowledge to improve your photos not as black and white rules for every shot. Rules != Creativity & Expression.
Some good advice above, and I'm a beginner myself so take this all with a pinch of salt. Out of the three I like the first the most, happy dog! My only comment is the foot is a bit distracting. I really liked the engine shot you had, nice DOF and arrangement.
I do try and keep composition in mind when shooting, but as you said sometimes it's just not possible.
Remember pictures are for you, thinking critically about what you are doing is really important, and only publish your best material. Consider having a go at Lightroom too for post-processing, it can really bring some pictures to life.
Take critique on-board when you want, it's very valuable to spot things you might have missed but photography is very subjective and photos should be for you first.
If someone tells you there are "rules" for photography and composition, break their camera!
, but i also like taking pictures for others lol, but like you, im still learning, and every comment is a plus for me
I've been taking photos since my uncle who was an army tog taught me 28 years ago, I've been shooting professionally for 5 or 6 years now, I am still a noob and still learning
I remember my first crit over on talkphotography, some people were brutal and it really bothered me but you learn to take it as an aide for learning (although some people are horrid just because its the internet and they can).
take part in the comps here, vote on them, give crit yourself. Doesn't matter you are new, you have an opinion of what works for you and what doesn't. Lots of noobs won't crit others but you should
