1. おなか、すいてる?
This is the most neutral among the three.
Asking "are you hungry" in a casual manner.
2. おなか、すいてるの? (I know it's that 「の」, but ...)
The の add some "softening" effect. Sounds milder than others.
Females tend to use this, though it sounds just mild, not necessarily feminine.
(And it is quite normal for male to use this.)
3. おなか、すいてるのか?
This sounds a bit rough somehow - because of か.
Same about すいてるか?.
Like "hungry, er?" perhaps?
Although か used with です・ます just convert the sentence into question, combination of dictionary form and か makes it sounds more than casual, a bit rough, or strong, tough.
Mainly used by male.
(This could sound demanding, or accusing, depending on the tone and context, too, though not always.)