When you suffix -하다 to 감사, it becomes a verb, and to 미안 and 죄송, they become adjectives.
미안요, 감사요 and 죄송요 are all shortened forms of 미안해요, 감사해요 and 죄송해요. They can't be abbreviated forms of 미안이에요, 감사이에요 (예요) , 죄송이에요 because they will have completely different meanings. For example,
Q: 칠판에 무슨 단어가 적혀 있지요? (Literally) What word is written on the blackboard?
A: 감사예요 (이에요). (감사란 단어예요/이에요) It's 감사. (It's a word called 감사)
Note: You can say "감사요" here, but it would never mean "Thank you".
미안요, 감사요 and 죄송요 are colloquial expressions you use in front of people you feel close enough not to have to use 미안해요, 감사해요 and 죄송해요. In other words, they are less polite forms than -해요 forms. Of course, -해요 forms are less polite than 미안합니다, 감사합니다 and 죄송합니다. As I mentioned in another post, the longer the sentence is, the politer it gets. Therefore, 미안합니다 > 미안해요 > 미안요 > 미안해 > 미안 in order of politeness.
One simple example is when you meet your colleague at work for the first time, you are more likely to use 미안합니다/감사합니다. After a few months, you are more likely to use 미안해요/감사해요. After a few months further after the acquaintance, you can use 미안요/감사요. When you and your colleague get close enough to talk down casually using 반말 (non-honorific words), you are more likely to use 미안해/미안/감사.
Basically, the above examples are too simplified. The usage might depend on your age and sex. The younger you are, the more often you will use the -요 forms. It is my impression that females use -요 forms more often than males.
I don't think they are slangs. I would call them colloquial expression or cute expression used between people who are close enough. In addition, you can use it on the internet post and nobody will take offense from it especially when you are female.
Note: I have never heard 사랑요 used in place of 사랑해요. But I don't see any reason not to use it if it makes sense between those who use them.