1. Some verbs plus 不着 means the negative forms.
E.G. 一喝晚上就睡不着觉。
我找不着我的钱包了。 I can't find my wallet.
我点不着火。 I can't light a fire.
2. What do you mean by 当累? Maybe you can add some English explanation.
The second sentence is correct.
3. 变得高兴 or 开心起来. If there is a context, we can give you more proper translation.