Does match 3 specifically use WebGl, if webgl is not available will it crash, through errors that are not caught etc. etc.
There is no reason why a match3 game should require WebGL - it will work fine in Canvas where WebGL is not supported.
That is, of course, if you don't use any WebGL-specific features like shaders to draw your match3 sprites.
The default renderMode is WebGl, Whether it is used or not.
Setting the renderMode to WebGl just means "use WebGl if possible, otherwise use canvas".
In most cases you want to keep the renderMode as WebGl, the only exception I can think of is if you are using vector graphics in your sprites draw functions (filling polygons with gradients etc).