Sorry for the late reply but I just saw your message now

A three day gap after a lively exchange usually means you were busy, not that you lost interest. The thing to check is what the conversation looked like when it stopped. If she sent a question and you left it hanging, she is likely wondering whether you are still in it. If the last exchange was a light sign off, the silence reads as normal life. Her message itself also gives clues. A playful tease with an open loop, like owing a drink, is an invitation to pick the thread back up. A flat one-word reply to your last message would mean she has already moved on and the apology needs to be briefer and lower stakes.

Also look at whether she has followed up on her own. If she sent a second message after a long gap, she is still interested enough to try. If she has not, you are not obligated to act like the silence never happened, but you should keep the reply easy for her to answer.

A short acknowledgement works best when it is followed by something that gives her an easy reply. One version is a brief honest line about being away from your phone, then a response to something she actually said. That keeps the focus on her and stops the apology from becoming the subject. A second version skips the explanation entirely and goes straight to the thing she raised, which works when the gap was small and the tone was already relaxed. A third version refers back to the earlier plan or joke so she knows you are picking up where things were, which fits when the conversation had a clear open loop.

What each of these should leave out is any sense that you are asking her to excuse you. Skip the long explanation, the second apology, and any line that makes her feel she needs to be forgiving. The reply should feel like you are back in the conversation, not like you are checking whether she is still mad.

Send the reply at a time that matches the rest of your conversation, not at 2am after a night out. A message that arrives late at night after days of silence can read as a reach rather than a normal catch up. Give it a few hours if she is quick to reply, or until the next day if the chat has been slow. If she answers, keep the energy matched and let the conversation do its own pacing.

If she does not reply within a day or two, send at most one short follow up that relates to something new, like a link or a question about something she mentioned. Do not send a string of messages asking whether she saw the first one. If that also goes unanswered, let it rest. Silence after two attempts is a clear enough answer, and continuing to message only makes the earlier delay look like a pattern.