New home settling concerns...
I know that new homes "settle" but it's only been 3 months... How much cracking, separation, etc. is normal for a new house and over what period of time should it be expected?
Hairline cracks in concrete are often shrinkage cracks and not a concern from a structural viewpoint. If no reinforcing was used in the concrete and/or insufficiernt ground prep was performed before pouring, it could get worse and need attemntion but nine time out of ten, it is nothing to worry about.
The wood showing gaps inside is less likely from 'settleing' that it is from the wood framing members drying and shrinking. Frame lumber is kiln dried to 19% surface moisture but may get rained on during construction. Paint and SR mud add a lot of moisture to a house during construction too. Then comes the dry winter months and the wood shrinks up. some modest gaps appear.
Only if it keeps getting worse, should you be concerned and pursue it. A little caulk and paint will fix it up once it stops shrinking, usually after one thermal cycle
You wouldn't buy a car that had poorly fitting doors and windows or unbalanced tires without the dealer fixing them first. Neither should you purchase a new house showing signs of needing touchup. Make it a condition of sale that he will take care of your concerns in a way that satisfys you. Hiring an experienced home inspector might help. It is iomportant to find an experienced one tho. Two week schools crank out diploms for this unregulated field for anyone who sits in on the classes so you don't want someone who just finished the training.