If you're using the current model financial regulations, 11.1(b) includes the requirement to use Contracts Finder for over £25k, 11.1(c) covers the OJEU for very large projects, 11.1(h) requires 3 quotations between £3,000 (usually) and £25,000, and 3 estimates below £3,000, 11.1(e), (f) and (g) set out the tender process, which applies to all contracts over £25,000 and these are reflected in 18(c), (d) and (e) of the standing orders.
To simplify all of that, 3 quotations are acceptable up to £25,000, but thereafter, tendering is required, in accordance with the arrangements set out in your standing orders and financial regulations.