Why can oxygen have a coordination number of 3 in a complex between silver and SiO2?
High coordination numbers are really not uncommon in solid-state compounds. For example, consider the structure of $\ce{Li2O}$, an anti-fluorite structure. In fluorite proper, calcium ions form a cubic closest packing structure and the fluoride ions occupy the tetrahedral voids. In $\ce{Li2O}$, it is reversed: the oxide ions form a cubic closest packing and lithium occupies the tetrahedral voids. That gives each lithium a coordination number of 4 and by maths and symmetry oxygen must have a coordination number of 8.

Tricoordinated oxide anions are also well-known in solid-state structures. The prime example would be the rutile structure of $\ce{TiO2}$ with hexacoordinated titanium and tricoordinated oxide ions.

In your specific case, the silver cations are acting as Lewis acids while the oxide anions (which have to lone pairs not directed towards neighbouring silicium atoms) can act as Lewis bases. A coordination number of three points to a somewhat covalent (and not so ionic) structure — which makes sense considering significant fragments of $\ce{SiO2}$ are still there.