First engagement photos

I have really only done landscapes and architecture - my experience doing portraits is very limited. Will I be okay with these lenses?
15-85 IS USM
50 1.8 prime
100mm 2.8 macro

Will I need to rent a 35mm?

Any other tips? What time of day is best? Start a couple of hours before sunset?

I don't have any lighting equipment.

Your current line up will be more than sufficient for an e-session. Obviously avoid harsh midday sun at all costs since you don't have lights. This time of the year I'd shoot around 6-9PM for some of the best light.

those lenses are fine; try the 100mm and the little 50. you might be amazed at what they can do.

try 50mm 1.8 prime or100mm 2.8 macro for the great sharpness

Use the 100 set at 2.8 as much as possible. Set them up and have them interact with each other while you grab the shots.

A couple hours before sundown is a great time to shoot. Make sure that your subjects are either in the shade or are between you and the sun as much as possible. The sun will make for a great hair light. Keep a dark background though so that everything isn't blown out.