There has been quite a bit of research surrounding mindfulness meditation for a couple decades now.

Here are some cursory studies addressing your three questions. They are easy to find on Google Scholar.

Decreased stress and anxiety http://online.liebertpub.com/doi/abs/10.1089/acm.2010.0142

Increased Focus (specifically for people on the ADHD spectrum) http://www.ahc.umn.edu/img/assets/20825/MindfulnessADHD-Zylo...

I have to imagine mindfulness is a difficult concept to disentangle from meditation itself, however there do seem to be attempts to create measures for this: http://journals.sagepub.com/doi/pdf/10.1177/1073191107313003

Here's a meta-analysis summarizing many studies about the effects of meditation. Effect sizes seem consistently positive albeit on the smaller side: http://link.springer.com/article/10.1007/s12671-012-0101-x