This kind of system could definitely host a planet given certain constraints.
A useful rule of thumb for long-term stability in a gravitational orbit is that all orbits must be separated at least by a factor of 7 in radius (see, e. g. Publications of the Astronomical Society of the Pacific, 115:825–836, 2003 July). For example, a planet could orbit at 1 AU from a star, and both can orbit a barycenter with another star which is at 7 AU a safety margin, use a factor of 10.
Having the planet orbit the single big star is easy. You can put it at 1 AU, and place the binary at least 10 AU away from the big star.
If you want to have the planet orbit a single star in the binary, you need a bigger system. You could have the planet at 0.25 AU from one star, have the binary stars orbit eachother at 2.5 AU, and have the third star and the binary orbit eachother at 25 AU.
Having the planet orbit the both stars in the binary is unfortunately very difficult. The problem is that the habitable zone of a 0.5 solar mass star is only 0.25 AU from the star (L~M^4). This means that to maintain the appropriate separation you would need the stars to orbit eachother at only 0.025 AU or around 5 solar radii. I think the stars can escape collision, but a system forming this way seems unlikely due to the tight constraints.
If you scaled all of the masses up to avoid this problem the big star would die relatively quickly (within a few billion years), which might have a disastrous effect on life developing on the planet.
If you want a circumbinary planet, you should probably have all the stars have the same mass as the Sun - then the binary could orbit at a more comfortable 0.1 AU and the planet could be placed at around 1 AU, with the third star much farther away.