hi my horse is lazy too and i use eathier a crop (whip) or dummy spurs (prince of wales spures) they work really well, well they did on my horse i hope it helps :)

 I think the first thing to look at is thinking in your head, woooooo I'm on a horse! Sounds ridiculous but just get yourself all geared up. I can get on a horse and it will be all jumpy and raring to go when usually its a donkey because when it comes to horses I have this internal joy and I feel like the horse feels it and goes, oh yay!
You haven't said what you're tried in regards to getting her a little bit more awake? Maybe she is bored of her surroundings?  Have you checked her over for any lumps or bumps, she could be feeling under the weather because something is wrong? Also she is only three. Maybe give her some time off to just be a horse, practise groundwork and getting her moving with your voice for a while and just general bonding. Then get back on when she's feeling refreshed and she may feel completely different. Good luck.