The combination of love and faithfulness is common (Ex. 34.6b; Ps. 86.15; 108.4; 115.1; 117.2; 138.2). If we take them, as I do, as human mirroring of God's attributes, love and faithfulness are good translations. "Mercy" is an older English term meaning love (used commonly in 1611, but often with a slightly different meaning in our era). I think the idea of the second part is trustworthiness, and hence faithfulness. Though "truth" is in the meaning, the idea imho is what we see in the other texts I mentioned, along with a text like 1 Cor. 13.13 (faith, hope, and love). In the OT, what was expected of the Israelites was covenant love for God (Dt. 6.4, and the "write them on the tablets of your hearts" even gives a nod to Dt. 6.6) and covenant faithfulness (loyalty and allegiance to the terms of the covenant). That's what I think best fits the verse. The way to God's heart (Prov. 3.4) was covenant love and faithfulness.

Proverbs 3:3 is a verse that inspires me a lot, but I have quite some confusion on the key terms of the verse. KJV say "mercy and truth," NIV says, "love and faithfulness," and ESV says " steadfast love and faithfulness." The original terms are חֶ֥סֶד and וֶאֱמֶ֗ת, but they seem to have several quite different definitions themselves as well. חֶ֥סֶד can mean Kindness, piety, reproof, beauty (Strong's Hebrew 2617). וֶאֱמֶ֗ת can mean Stability, certainty, truth, trustworthiness (Strong's Hebrew 571). Which definitions do you think fit this verse?