Hey TZ22. Sounds like you have maybe cut back a little too much. 7 weeks out to lose 3kg is definitely doable for most. A wee word of advice is to be careful about cutting out meals - particularly this early on. Absolutely, drop the sweets, sugar and cream in your coffee, alcohol if you drink, watch salt intake too but unless you are eating crazy amounts I wouldn't drop meals - particularly if you are training 6 x week. Listen to your body. You need to fuel it - one of the main reasons people can't lose weight when exercising is because they are not eating enough. If there is not enough fuel going in to the body to come close to the out put of energy your body will go into survival mode, effectively starvation mode. It will work extra hard to hold on to all of it's reserves. The basic formula is to ensure your calorie intake is less than your energy spent - however - it shouldn't be a huge difference - just enough to put you in a deficit that still allows your body to function at a high level and to sustain that deficit for as long as needed. Over time (usually 6 to 12 weeks) your body will re-calibrate it's 'normal' weight. There is truth in the old saying if you lose the weight slowly it will stay off.
My fitness pal or a similar app can help you keep track of your levels so you can stay on track for a sustained cut so you don't need to dehydrate.
Few tips - make sure you are drinking enough water. If your body is dehydrated it cannot burn fat. It's in survival mode. It's something ridiculous like 1% dehydration leads to a steep decrease in fat burning.
Don't worry so much about a few lbs change on the scales over night or during the day. Water weight fluctuates daily so it's normal to have lost slightly and then regain and vice versa. Weigh yourself once or twice a week on the same scale, in the morning, after using the bathroom and keep a note. You will see the trend. Low salt intake will help with water retention, as will eating refined carbs and simple sugar such as fruit.
In all honesty, I reckon if you eat balanced for 7 weeks, keep hydrated, cut out refined sugar and keep the salt low you will smash the target easily if you are training 6 x week. Don't neglect good sugars - fruit etc - you need them!
Best of luck on it - I am sure you will be grand :)