To determine which issuer a bank should engage first for a hybrid bond transaction, we must evaluate the entities based on criteria relevant to hybrid capital issuance: size/financial strength, existing familiarity with hybrid instruments, and appetite for growth/capex requiring flexible capital.

1. **Entity A (TERNA S.P.A.)**: Terna is a massive infrastructure company (Assets: €22.8B) with significant capital expenditures (€1.7B in investing activities). Crucially, Terna already has hybrid bonds on its balance sheet ("Equity Instruments Perpetual Hybrid Bonds" of €989M), demonstrating an established framework, investor familiarity, and an active appetite for this exact instrument to fund its expansion. This makes Terna the most promising and straightforward prospect for a new hybrid bond transaction.

2. **Entity C (REDEIA CORPORACION SA)**: Redeia is also a large infrastructure entity (Assets: €14.8B) with heavy investing activities (€2.3B outflows). While it doesn't currently list hybrid bonds as a standard line item, its cash flow statement shows massive recent activity in equity instruments ("Cobros Pagos Por Instrumentos De Patrimonio" of €989M and "Proceeds From Sale Or Issue Of Treasury Shares" of €1B). Furthermore, its statement of changes in equity shows "Otras Operaciones Con Socios O Propietarios" (Other transactions with shareholders/owners) of €960M. This strongly suggests Redeia is actively utilizing alternative/quasi-equity instruments or hybrid-like capital structures to fund its growth, making it a highly viable second target.

3. **Entity B (A2A ENERGIA S.P.A.)**: A2A is smaller (Assets: €21.3B, though heavily skewed by short-term energy trading receivables and inventories) and more volatile, evidenced by its revenue doubling and then massive swings in working capital. It relies primarily on traditional bank borrowings and standard bonds, showing no signs of hybrid capital usage or a dedicated equity/hybrid framework in its financial statements. It is the least suitable and least promising of the three for a hybrid bond origination.

Prioritizing from most promising to least: Entity A, Entity C, Entity B.

<answer>A,C,B</answer>