To my procurement colleagues: how do you procure 3PL spend in your companies? is it centralised (i.e., regionally, globally), and if yes, is the centralisation valid for all contract values or you have some thresholds? 

We have this as a regional category, so strategy is set based on geographical scope. However when we go out to market it is typically at a level where there is comparable competition e.g. UK, mainland Europe etc. Ocean freight and associated control towers are globally managed. Across all of logistics we have a small number of global strategic partners who may operate in multiple regions / multiple sub-categories (e.g. warehousing, transport, control towers, ocean) and those global relationships are managed globally.
Hope this helps.

For us we manage our Logistical spend more collaboratively between Supply Chain, Manufacturing and Procurement. Structurally - Procurement owns/develops a global category strategy that is aligned with input from Supply Chain primarily. Operationally, except for global 'movement', such as Ocean cargo we manage regionally by spend sub-category - Warehousing and the various modes we utilize from TL to final mile delivery (Fleet, Courier and LTL) - leading to bids, annually for Transportation and multi-year for warehousing given asset investments required for warehousing & systemic integration. Not all contracts are managed centrally in Procurement - generally defined by spend threshold OR if a strategy over-rides that where more central expertise is required.