Praise be to Allah.
Delaying prayer due to work
“Men whom neither trade nor sale (business) diverts from the remembrance of Allah (with heart and tongue) nor from performing As-Salah (Iqamat-as-Salah) nor from giving the Zakah. They fear a Day when hearts and eyes will be overturned (out of the horror of the torment of the Day of Resurrection).
That Allah may reward them according to the best of their deeds, and add even more for them out of His Grace. And Allah provides without measure to whom He wills.” [Al-Nur 24:37-38]
So you must organize your time at work in a way that will not interfere with your performing the prayers on time and work out an agreement on a suitable solution with the administration, even if that causes you some difficulty, such as working extra hours, for example. Note that the benefits of increased faith in your heart that you will get as a result of praying on time will compensate for any difficulties you encounter, and that hardship will turn to joy – in sha Allah – because you will be putting up with it for the sake of Allah and seeking His pleasure.
Something that the questioner deserves to be appreciated for is his regret at having missed prayers and having prayed them at the wrong time. This is how the believer should be, he should feel regret if he misses out on some righteous deed. But this regret should be a motive for him to correct his actions and avoid falling short in them. If he feels regret in his heart yet he still persists in missing prayers and doing bad deeds, then this is not right.
Can you joining ‘Asr and Maghrib?
You say that you join ‘Asr and Maghrib prayers before the start of your shift or afterwards.
You should note that the prayers which are allowed to be joined according to Shari`ah are Zuhr and ‘Asr , or Maghrib and ‘Isha. These are the combinations mentioned in Shari`ah. As for joining ‘Asr and Maghrib, this is not narrated in Shari`ah and is not valid. This was not suggested by any of the scholars. Based on that, if you have joined ‘Asr and Maghrib after sunset, you have to repent to Allah for delaying ‘Asr prayer until its time was over, and you have to resolve not to do such a thing again.
If you joined ‘Asr and Maghrib before the time for Maghrib began, which is sunset, then you should note that prayers done before their time begins are not valid, so this Maghrib prayer of yours is not valid. Based on this you have to figure out the number of times that you prayed Maghrib before its time began – if you are uncertain then you should be on the safe side and assume it is the higher number – and repeat those prayers. Hasten to do that as much as you can.
Do your best not to delay the prayer
You must try hard to solve this problem. It will not take you more than ten minutes to do this thing, so you can agree with the boss that you will make up this time before or after your shift. You may not be able to imagine that you can ask permission for ten minutes from work, but if you wanted to go to the bathroom they would never stop you from doing that, even though that may take the same amount of time or more. In your country there may be laws that protect the rights of minorities to practice their religion and which oblige bosses to respect the religion of those who work for them. You may have such laws which will enable you to ask for your rights.
If it is too difficult for you, and you cannot solve this problem with the administration, then you have to look for another job which will not conflict with your establishing regular prayer. If you cannot find any other job and you will be adversely affected by your leaving this job, then there is the hope that this may be regarded as a necessity that will permit your joining your prayers and there will be no sin on you in that case, in sha Allah.
You can join Zuhr and ‘Asr, at the time of either of them, and you can join Maghrib and ‘Isha at the time of either of them, depending on what is easier for you.
We ask Allah to help us to understand Islam and to do and say good things, and to make things easy for you.
And Allah knows best.