A couple days into flowering a small plant. Can I top or fim her?

She's a couple of weeks old, about a foot high and 3 days into flowering. What would topping or fimming her do now? would I be better of without doing anything?

Yes, you can top or FIM now and for another week, maybe two. You can also do it a few weeks before flowering as well. The point of doing either of these techs is to increase the top (dominant) buds...topping should double the tops, FIMing can give multiple tops (2 to 6!) if done correctly.

does it really make an increase in yield, or does it only look that way? have you tried comparing?

Topping/fimming may increase the yield if done correctly in vegging stage. After that needs to be vegged longer. Because plants need to recover after that stress. And new development takes one to two weeks, in total needs to be vegged two months approximately to get some good result.
If they're already in flowering it will not increase the yield. It will just look that way because you'll not have a main cola and energy will be spread to the others and some will be lost by recovering. In the end visually can look good but if you have place to go, go without from now on. If you're limited, you can consider some LST. Simply tie down the main cola from the last 2nd or 3rd inter node. Can be hard if you try to do from the lower inter nodes if the stem is thick. You can keep training for a week or more. Whenever the main bud straights up, tie down again. This usually happens just in a day. When you get most of the colas on same level you can stop training. Check my log, I just did a simple LST on second week of flowering. Can give you an idea.
I previously had two plants, one without training and one with LST. Results looked same to me. I didn't see any noticeable difference except for all visible buds on top with LST. Other without training had the same buds just somewhere in lower place. But they were different strains i must tell.

I would leave it alone and flower on it's own. After 2 weeks into flowering I normally would take off the bottom branches that are not flowering much. Always support your plants main branches and tie plant into "S" shape. I would also give it Deuce-Deuce made by Humboldt Nutrients when the plant is 4 weeks into flowering.

Man, I gotta stop posting when I'm too buzzed, I ALWAYS forget stuff!...Sorry Kim if I misled you...yes, you can top or FIM, and it will give you more dominant top buds, however, they will not be quite as large as if you had left them alone, there will just be more of them...I'm starting to think just tying them or bending them down to fool them into making more dominant "top" buds may be the way to go for me, it just HAS to be less stressful to the plant imo...There is a debate on whether any of this stuff actually increases yield or not. Last batch I LST'd, trained and staked and removed almost all undergrowth...I got 6-7 very large buds off each plant, but since I did no comparison, I don't know if it really made the buds bigger or not. They were bigger than the previous batch, (and I use clones, so the plants were exactly the same) but I also added molasses to the grow this time and it could have been the difference, so who really knows for sure? Do one and not the other and see, maybe?