I would think she is allowing herself to be stroked to climaxes, but is not allowing penetration just yet. There is also the possibility that Caleb and Matt have their own supply of condoms. But she has been out for dozens of nights now with her friends, and you are continuing to encourage her, and tease her.
What is holding her back? She is worried about losing her husband. I would think, because she knows that once she "goes all the way", that she will find it impossible to stop it happening again and again. She is already aware that she is walking on the "edge" of adultery.
I think you need to tell her that she now has three men in her life, and your marriage is SO MUCH HAPPIER with her FEELING YOUNGER having college fun with young guys still at school.
You need to tell her that your love for her has become much stronger since she started dating Caleb and Matt. That you love buying her clothes so she can feel beautiful on her dates; and you like her going out without a bra or panties to have a very sexy time.
Tell her that you are very pleased that both Matt and Caleb have seen her naked and are getting eyefuls and a chance to feel her up down below.
Tell her you are very proud of her for helping Caleb overcome his shyness with girls, and by letting him touch her body all over and kiss her passionately, she is helping him to learn about how to have sex with a future wife. If she teaches him well, then he will have a happier marriage, thanks to Amber's effort.
Tell her that she is doing a wonderful job by letting Matt and Caleb see her pee in the toilet and get plenty of eyefuls in the shower. Say that you wish you had had a teacher to help you learn about sex when you were growing up.
Also, tell her that the condoms are for her to use on every date she goes out on. That these will stop her getting pregnant accidentally.
Tell her that you want her to take Caleb's virginity to help him become a man - because he deserves to be properly taught by a beautiful woman like you.
Tell her that fucking Caleb is a "special gift she can give him" to show that she enjoys his company, and also appreciates the time he spends with her at parties, etc.
Tell her that you would like her to have extra sex to make her feel younger and "more desired". Three men (including you) think she is a wonderful person, and all three of you want to spoil her like a filmstar.
Tell her that you want her to have extra sex with these younger men, because it is your gift to her for being such a wonderful wife to you.
Tell her that you will become very excited and pleased for her when she comes home, having used at least one condom, that it will feel as though she is showing you that she loves you more than ever.
Tell her that you would like her to come home with a snail trail over her breasts, and you will lick it up to prove that you are okay with what has happened. That will play on her mind and she will discuss it with her best friend who will tell her to "go for it, because it is your wish for her".
In her mind, her wedding vows are sacred, but she is beginning to realise that you are giving her enough space to include more sex in her life by altering the terminology of what sex is (kissing, touching, tit sucking, sleeping nude, eyefuls, letting guys see her pee), expanding out finally to oral and full penetration and ejaculation.
The most important thing is to keep her going out on dates. The guys are shy, but are slowly expanding her boundaries, and every date that goes by means more and more of her body is being made available to them. Caleb pushed her, then suddenly Matt was allowed to see more skin.
I would like to see her get nude massages to get her wet between the legs more often.