Final answer:
The figure of speech in Psalm 71:3 is a metaphor, with God being likened to a 'rock' and a 'fortress' to imply strength, protection, and reliability.
Step-by-step explanation:
The figure of speech in Psalm 71:3 from the King James Version is metaphor. This is a figure of speech where a word or phrase is applied to an object or action to which it is not literally applicable. In this verse, a metaphor is used when God is referred to as a 'rock' and a 'fortress'. This implies that God is a symbol of strength, safety, and security, in the same way that a rock is solid and unchanging, and a fortress offers protection. The use of metaphor here is intended to convey the steadfastness and reliability of God as a refuge to the believer, offering a place of safety and shelter from life's troubles.