They are, in fact, quite different - yoga moreso than the other 2.

Yoga, firstly, isn't a Chinese exercise - it comes from the India (and surrounds). It is part meditation (if you get good enough at it), and a lot of stretching of muscles/tendons. You stretch your body, maintain a position, and try to meditate. In the early stages, it can be painful for beginners (who have a bad tendency to over-stretch themselves... literally!)

Taiji is a much gentler exercise, comparitively, and many different forms exist. Some are quite slow (moving with a slow inhale and exhale of the breath), some can be fairly fast (more towards the martial applications). You don't need to stretch the same way as yoga, and while it is a form of meditation, it is meditation while moving. It is designed to gently balance the energies (qi and blood, yin and yang) by 'opening' the channels (ie, stretching them out a bit, get things moving, etc). It's movements tend to be circular, and constantly changing directions. The more martial version of taiji is closer to kungfu, so should probably (in a non-traditional sense) be considered as such.

Qigong is similar in philosophy to taiji, but is a much 'stronger' version. It also has many different styles. It tends to have more sharper, faster movements than taiji, to build a much stronger body. In theory, one is building one's energies to be 'tougher' and to be able to withstand attacks (physical and mental) better and faster. And that's one of the major differences between taiji and qigong - in qigong, there is an expectation that you will develop that inner strength much faster than in taiji - because it's tougher.

That's a pretty basic (and no doubt in some ways, unsatisfactory) answer, but since you seem to be very new at these, it may help a little.

So, what are looking for??