있는데 means when you want to talk something in ur mind to somebody, you use it and also when you want to inform of something somebody who have not recognized it. 있는지 means when you are not sure about something exist or not, you use it. it's kinda "i'm wondering that~" 있을까 means almost same as above word "있는지" though. it is used as question. so it accompany question mark. i hope you understand its words in sentences.