The ideas you've listed thus far seem exauhstively descriptive and borderline robotic.
A tagline should be as short as possible, and preferably not formal, or as I tried to put it, exauhstively descriptive.
The best process as far as I'm concerned is to think of what exactly makes your application useful, or sepparates you from competitors, which in this case are likely just traditional methods of getting a cab, and find a very succinct statement that gets that idea across in a very human (not so formal) way. Even if it's gramatically incorrect, it doesn't matter. The phrase should add some sort of branded personality to your product. Much like a logo would add visual recognition and connect some less tangible ideas or emotions to the product or company, a tagline should do the same but in words, rather than graphics.
The three you've mentioned seem more like a bland headline or openning to an advertisement to simply state what you do. The connection is direct enough that it doesn't add any recognizability.
Dominik Oslizlo's comment above, "Search. Tap. Go." is on target. Whether or not this is one you like personally, it works as a tagline. It's informal, not exauhstive, and connects an informal, but descriptive phrase to the product, without blandly describing exactly what it does.