Hmm, sorry if I am not being clear. Let me be clear: semantically speaking, there's no reason your argument couldn't be true. If your question is, "could this rule be interpreted this way" then the answer is "yes you totally can interpret the rule this way and the Sergeant can have 3 items". In this way, yeah, you got it covered: your interpretation can be correct. However, in almost every sense other than this very constrained literal one, your interpretation of the rule is wrong.
What do I mean by this? Well, in my opinion we should ask a different question than "could this rule be interpreted this way". The question we should ask is: how is the game played, and understood, by everyone who interacts with this rule, when acting in a reasonable way? And furthermore, when this rule is in use, what's the meaning that you can convince others it means? I think that my question is relevant because ultimately, the rules have no meaning except insofar as you and your gaming buddies interpret them when you play with each other.
The question here is actually a question of RAW. As intended, if we assume the intent has to do with how Sergeants used to be, the Sergeant is meant to have 2 weapons. But what about as written? So we must ask, what does "replace" literally mean, as written? Replacing can mean 1-to-1 replacement in a strict way, and this is how it is usually meant (replace a light bulb, for examle). But in day to day usage we might also say that we replaced a sandwich with a pile of chips, and this isn't incorrect either. So in fact, this RAW question comes down to the definition of a word, which can be contentious. What matters, then, is how the word is understood and how it can be made to be understood.
Perhaps this particular philosophy of approaching the rules seems sloppy or strange to some people. However, my philosophy is that I want to actually be able to play this game with people. So, when we encounter strangeness in the rules, we go with the socially-agreed rule that is possible to convince people of. If someone is wrong on the rules in a demonstrable way, that's an opportunity for us to open the book and convince them: but ultimately, on something like this, what matters is that a reasonable person would be convinced that this rule makes sense.
I think you would be hard-pressed to convince someone that the sergeant who can "replace his bolt pistol and boltgun for items..." can replace them for three items. I think that if you have to sit down and argue the definition of replace and say "well, there's no reason you can't replace 1 item with 2 items" and give it a go. But I think most people will disagree with you, and you won't be able to play using this rule.
In my opinion, unless otherwise specified, RAW, replacement is 1-to-1 for wargear. I don't find your arguments to the contrary to be compelling. I don't think other people will find them compelling either, which means insofar as actually playing the game goes: A sergeant can have 2 pieces of wargear from the Sergeant Equipment list.
I'm not saying you're trying to convince anyone. I'm just saying that, at some point, you will need to, if you go with this interpretation of the rules, and it is unlikely they will find this argument convincing. This matters.